Professor  A.  0.  leuschner 
1868-1953 


Gift  of 
Dr.Erida  Leuschner  Reiche: 


EARTH  EVOLUTION 

AND  ITS 

FACIAL  EXPRESSION 


BY 

WILLIAM  HERBERT  HOBBS 

CHARACTERISTICS  OF  EXISTING   GLACIERS 
EARTH  FEATURES  AND  THEIR  MEANING 

SIMPLE   DIRECTIONS  FOR  THE  DETERMINATION  OF 
THE  COMMON  MINERALS  AND  ROCKS 


EARTH   EVOLUTION 

AND  ITS 

FACIAL  EXPRESSION 


BY 
WILLIAM  HERBERT  ,HOBBS 

Professor   of   Geology  and   Director  of   the   Gv6logical  Laboratory, 

University  of  Michigan 

Author   of   "Earthquakes,   an   Introduction    to   Seismic    Geology"; 
"Characteristics  of  Existing   Glaciers";   "Earth   Features 
and  Their  Meaning";   "The   World  War  and  Its 
Consequences";    "Leonard    Wood,    Ad- 
ministrator,   Soldier,    and 
Citizen,"  etc. 


Jl3eto 

THE  MACMILLAN  COMPANY 
1921 


All  rights  reserved 


PRINTED  IN   THE   UNITED   STATES   OF  AMERICA 


COPYRIGHT,  1921, 
BY  THE  MACMILLAN  COMPANY. 

Set  up  and  printed.     Published  October,   1921. 


REP.  «EN.  LIB. 
NO. 


GIFT 


Press  of 

J.  J.  Little  &  Ives  Company 
New  York,  U.  S.  A. 


TO 
CHASE    SALMON    OSBORN 

One  time  Regent  of  the  University  of 
Michigan  and  Governor  of  the  State,  lover 
of  Nature  and  versed  in  the  secrets  of  the 
wild-life  of  field  and  forest,  discoverer  of 
great  iron  deposits,  world  traveller,  lion 
hunter,  editor,  author  and  friend,  this  volume 
is  dedicated  by 

THE  AUTHOR 


W88S863 


INTRODUCTION 

To  the  structure  of  the  body  of  scientific  thought  the 
nearest  parallel  is  found  in  that  of  a  well  constructed  edifice, 
with  each  part  so  designed  as  to  fit  accurately  and  without 
intervening  space  to  other  parts  in  its  neighborhood,  and 
all  reared  upon  a  common  foundation.  Were  our  knowledge 
perfect  this  ideal  might  be  realized  and  no  harm  result,  but 
we  are  accustomed  to  erect  the  superstructure  over  the  ac- 
cepted fundamental  tenets  of  the  science  of  our  time,  just 
as  though  history  did  not  warn  us  that  on  the  long  upward 
course  of  the  growth  of  knowledge  the  dominant  thought 
of  one  period  has  often  been  abandoned  in  the  next. 

A  survey  of  the  past  will  show  that  the  great  advances 
in  scientific  thought  have  followed  the  discovery  of  new 
points  of  view,  viewpoints  which  have  offered  new  outlooks 
over  the  field  ahead.  Such  a  survey  will  tell  us  further  that 
our  mistakes  in  the  past  have  arisen  very  largely  because 
from  each  new  coign  of  vantage,  as  it  has  been  attained,  the 
attempt  has  been  not  only  to  adjust  the  nearer  field,  but 
to  sketch  in  strong  lines  the  hazy  distances  as  well  as  the 
nearer  landscape.  Better  than  tarrying  so  long  at  each  sta- 
tion would  it  have  been  to  advance  over  the  field  already 
mapped  so  as  to  secure  a  better  viewpoint  and  bring  up 
the  distant  horizon.  Our  etapes  have  been  both  too  long 
and  much  too  infrequent. 

If  the  foundation  of  our  structure  is  to  be  removed,  it 
becomes  necessary  for  the  superstructure  to  be  taken  down, 
and,  before  rebuilding,  such  material  as  can  be  used  from 
the  old  structure  must  be  fitted  to  the  form  of  the  new 
basement. 

Far  more  than  is  generally  supposed,  the  recent  aban- 

vii 


viii  INTRODUCTION 

donment  of  the  nebular  hypothesis  to  account  for  the  origin 
of  the  universe,  must  carry  with  it  a  rewriting  of  our  science. 
This  is  particularly  true  of  geology  for  all  that  concerns 
seismology,  volcanology  and  the  whole  subject  of  the  growth 
of  continents  and  mountains.  The  present  tune  is  there- 
fore an  opportune  one  for  supplying  to  these  fields  a  dis- 
cussion which  can  be  harmonized  with  the  newer  viewpoint 
and  be  independent  of  the  abandoned  groundwork  of  the 
science. 

For  nearly  a  score  of  years  the  author  has  concerned  him- 
self with  certain  of  the  fundamental  questions  of  theo- 
retical geology  which  are  in  one  way  or  another  connected 
with  the  growth  of  continents  and  mountains,  and  these 
studies  moving  along  different  directions  were  converging 
upon  important  results  when  they  were  interrupted  by  the 
distractions  of  the  World  War.  Following  the  war  period 
the  studies  were  resumed,  and  some  of  the  results  have 
already  been  communicated  in  technical  papers  read  at 
recent  meetings  of  the  Geological  Society  of  America,  the 
American  Philosophical  Society,  and  the  Association  of 
American  Geographers.  To  the  non-technical  reader  with 
a  background  of  general  reading  only,  the  matter  of  these 
papers  must  be  presented  in  somewhat  different  language, 
and  much  must  be  brought  in  from  those  earlier  studies 
which  are  now  scattered  through  technical  journals  both 
in  this  country  and  abroad,  but  which  have  paved  the  way 
for  the  later  conclusions. 

The  author  feels  that  he  has  perhaps  now  reached  that 
age  and  experience  where  he  is  warranted  in  venturing  out 
upon  a  sea  never  accurately  charted,  where  there  have  been 
many  shipwrecks,  and  where  there  certainly  will  be  many 
more  before  the  most  closely  guarded  secrets  of  the  universe 
have  been  disclosed.  He  cannot  hope  to  succeed  where  many 
others  have  failed  unless  he  avails  himself  of  the  knowledge 
and  experience  gained  in  the  past,  while  refusing  to  be 
hampered  by  any  rules  which  seem  to  him  to  be  based  on 


INTRODUCTION  ix 

false  premises,  however  exalted  may  be  the  esteem  in  which 
they  are  generally  held. 

The  views  of  the  evolution  of  our  planet  and  the  expres- 
sion of  its  face,  as  these  are  set  forth  in  the  following  pages, 
we  in  noteworthy  degree  a  result  of  the  independent  study 
and  well  considered  opinion  of  one  who  has  already  for 
thirty-five  years  given  his  attention  to  many  phases  of 
geological  study.  If  they  supply  a  viewpoint  which  can 
be  made  of  service  in  mapping  a  new  area  of  the  great 
unknown,  the  object  of  their  publication  will  have  been 
secured. 

Ann  Arbor,  Michigan, 
April  4,  1921. 


CONTENTS 


CHAPTER 

INTRODUCTION 


I  THE  NEBULAR  HYPOTHBSIS  AND  THE  SUPPOSED  EARTH  CRUST  .  1 

II    THE  NATURE  OF  THE  EARTH'S  INTERIOR 12 

III  THE  SOURCE  OF  VOLCANIC  LAVA 28 

IV  THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK     . 44 

V    THE  DEPTH  OF  THE  FOLD  POCKETS 63 

VI    THE  VAPORS  AND  GASES  OF  LAVA 72 

VII  THE    CHANGES    OF    FIGURE    WHICH    THE    EARTH    HAS    PASSED 

THROUGH 81 

VIII    THE  PRESENT  REGIONS  OF  RAPID  CHANGE 95 

IX    THE  CONTRASTED  ASPECTS  OF  THE  EARTH'S  FACE Ill 

X    THE  MIGRATION AL  MOVEMENTS  OF  THE  SURFACE 119 

XI  THE  PATTERNS  OF  THE  FACIAL  WRINKLES  AND  THEIR  MEANING  135 

XII    THE  DESIGN  OF  THE  FRACTURE  MARQUETRY 152 

XIII  LAVA  COMPOSITION  IN  RELATION  TO  EARTH  PHYSIOGNOMY     .     .  159 

XIV  EARTH  THEORIES  IN  RETROSPECT          174 


LIST  OP  ILLUSTEATIONS 

PAGE 


I     (Frontispiece),  Plan  of  the  earth's  face 

II  Effects  of  earth  movements  upon  rails  and  bridges.  1,  Rails 
buckled  in  approach  to  bridge,  earthquake  of  1891,  Japan. 
2,  Nagara  Gawa  railroad  bridge  dropped  into  river  but  with 
all  spans  still  connected,  earthquake  of  1891,  Japan  ...  48 

III  Effects  of  earth  movements  upon  rails.    1,  Buckled  rails,  earth- 

quake of  1888,  Charleston,  S.  C.    2,  Buckled  rails,  earthquake 

of  1879,  India 48 

IV  Effects  of  earth  movements  upon  rails  and  bridges.     1,  Buckled 

rails,  earthquake  of  1906,  California.     2,  Bridge  with  girder 
under-ridden  by  abutment,  earthquake  of  1891,  Japan     ...       48 

V  Effects  of  earth  movements  upon  bridges  and  curbing.  1,  Dis- 
torted abutment,  Salinos  river  bridge,  earthquake  of  1906, 
California.  2,  Buckled  curbing,  earthquake  of  1906,  Cali- 
fornia  48 

VI  Experiment  for  producing  arcuate  wrinkles  by  underturning  of 
plastic  Canada  balsam  during  the  contraction  of  a  rubber 
sheet.  1,  The  apparatus.  2,  The  effect  on  the  balsam  ...  124 


FIGURE 

1  Diagram  to  illustrate  a  theory  of  the  constitution  of  the  earth       18 

2  Diagrams  to   illustrate   the   different  theories   concerning   the 

earth's  interior 19 

3  Structures  of  meteorites.    1,  olivine  crystal  from  the  Pallas  iron 

on  which  the  oval  areas  are  the  only  remnants  of  the  crystal 
faces;  2,  chondrule  with  veinlets  of  rock  glass;  3,  chondrules 
with  eccentric  spherulitic  structure 21 

4  Composite  rock  diagrams  for  the  main  classes  of  rocks     .     .       35 

5  Sample  diagrams  to  show  the  composition  of  lavas    ....       36 

6  Sample  diagrams  to  show  the  composition  of  individual  shales 

and  slates 37 

7  Maps  to  indicate  how  a  characteristic  petrographic  province 

may  develop  from  local  fusions  of  shale 41 

8  Diagrams  to  illustrate  the  effects  of  folding  and  of  faulting  of 

different  types  upon  the  superficial  area  of  the  earth     .     .      46 

9  Section  across  a  normal  fault  showing  how  it  may  increase  the 

surface  area  of  the  earth 47 

xiii 


xiv  LIST  OF  ILLUSTRATIONS 

FIGURE 

10  Diagrams    to    show    how    block    faulting    may    yield    a    lava 

chamber 49 

11  Model  showing  the  crushing  of  a  shale-like  member  beneath  an 

anticline  of  a  competent  limestone-like  formation      ...       50 

12  Diagram  to  show  position  of  a  magma  chamber  formed  under 

a  competent  formation  in  an  anticline 51 

13  Successive  diagrams  showing  the  manner  of  contraction  of  a 

magma  chamber  through  continuation  of  the  folding  process 
with  expulsion  of  the  lava  through  a  conduit 52 

14  Contrasted  views  of  the  origin  of  laccolites 54 

15  Section  of  the  laccolite  of  the  La  Plata  Mts.  (after  Holmes)     .       55 

16  Contrasted  views  concerning  the  origin  of  laccolites    ....       55 

17  Map  of  the  laccolites  of  the  Judith  Mts.,  which  are  formed  be- 

neath  a   competent   limestone    formation    (after   Weed   and 
Pirsson) 56 

18  Map  of  the  Black  Hills  laccolites  formed  beneath  a  dome  in 

competent    limestone    (after    Jaggar)        57 

19  Section  of  the  laccolite  of  the  La  Sal  Mts.  with  included  shale 

(after  Holmes) 57 

20  Generalized  section  across  the  Andes  showing  compressed  lac- 

colitic  domes  (after  Steinmann) 58 

21  Map  of  the  arcs  at  the  front  of  the  Rocky  Mts.  with  their 

cores  of  intrusive  igneous  material 60 

22  Manner  of  formation  of  a  syncline  in  a  marble  slab  under  its 

own  weight  alone 67 

23  Anticlines  produced  by  the  pressure   (shove)   of  a  continental 

glacier.    A,  in  Queenstown  shale  on  Lake  Ontario  (Kindle)  ; 

B,  in  coal  bed  in  Illinois  (Sauer) 67 

24  Sections  of  tilted  coral  reefs  on  islands  of  the  Dutch  East  Indies 

(after  Verbeek)        69 

25  Successive  positions  of  anticline  forming  in  a  rising  island  arc 

(after  Brouwer)        70 

26  Manner  of  formation  of  elevated  reef  at  front  and  of  barrier 

reef  at  back  of  a  developing  anticline  arc 70 

27  A  tetrahedron  with  bulging  faces  and  truncated  angles  to  bring 

out  its  contrasted  antipodal  relationships 82 

28  The  earth  expressed  as  a  departure  of  the  spheroid  toward  the 

tetrahedron 83 

29  Series  of  polyhedrons  from  the  sphere  with  greatest  volume  to 

the  tetrahedron  with  least  volume 87 

30  Generalized  expression  of  the  earth's  figure  at  the  close  of  the 

first  great  era  of  geological  history 90 


LIST  OF  ILLUSTRATIONS  xv 

FIGURE  PAGE 

31  The  continents  at  the  end  of  the  Paleozoic  era  (after  Arldt)     .       91 

32  Generalized  expression  of  the  earth's  figure  at  the  end  of  the 

second  great  era  of  geological  history 92 

33  Generalized  expression  of  the  present  figure  of  the  earth     .     .       93 

34  Map  on  Molhveide  projection  to  show  the  distribution  of  his- 

toric earthquakes  for  the  land  areas  of  the  globe   (after  de 
Montessus) 96 

35  Map  on  Mollweide  projection  to  show  the  distribution  of  world 

earthquakes  for  both  the  land  and  water  areas  of  the  globe 
(after  Seismol.  Comm.  Brit.  Assoc.  Adv.  Sci.)      ....       97 

36  Diagrams  to  show  the  form  of  the  sections  of  rising  arcs  (after 

Staff) ; 98 

37  The   arcs   of   eastern  Asia   with   their  fore-troughs  and  their 

fringes  of  volcanoes 99 

38  Map  of  the  western  hemisphere  to  show  the  continental  shelves 

(from  Andree's  Handatlas) 101 

39  Map  of  the  region  about  the  Arctic  Ocean 102 

40  The  form  of  coast  terraces 103 

41  Terraced  coasts  of  recent  uplift  about  the  Pacific  Ocean     .      .      104 

42  The  dated  step  in  a  coastal  staircase  in  Alaska  (after  Tarr  and 

Martin) 105 

43  Diagram  to  illustrate  the  relation  of  a  progressive  settlement  of 

the  ocean  floor  sector  to  the  elevated  Pacific  strands  .     .     .     106 

44  Diagrams  to  explain  the  paradoxical  compression  within  rising 

mountains 107 

45  Comparison  of  raised  terraces,  Pacific  and  Atlantic  (from  data 

by  Tangier-Smith  and  Barrell) 108 

46  A  sample  geological  section  showing  in  shading  the  part  re- 

moved by  erosion 112 

47  Map  to  show  the  contrasted  aspects  of  the  earth's  face  within 

the  Eastern  Hemisphere   (after  Suess) 114 

48  Map  to  show  the  relation  of  the  Permian  mountains  of  Europe 

to  those  of  America 115 

49  Earlier  (incorrect)  and  later  (correct)  methods  of  representing 

mountain  systems  upon  maps 120 

50  Map  of  Java  with  fore-troughs  (after  Brouwer  but  with  addi- 

tional data) 121 

51  Geological  section  across  Java  (after  Yerbeek  and  Fennema)         122 

52  Sketch  map  of  southern  Europe  to  show  the  position  of  the 

"Bohemian  Mass"  with  reference  to  the  mountain  arcs  123 


xvi  LIST  OF  ILLUSTRATIONS 

FIGURE  PAGE 

53  Sketch  map  of  southern  Asia  to  show  the  position  of  the  "In- 

dian Mass"  with  reference  to  the  mountain  arcs     ....      123 

54  Diagrams  to  illustrate  the  contrasted  views  of  Suess  and  the 

author  concerning  the  direction  of  the  active  thrust  in  moun- 
tain evolution.    A,  surf;  B,  Suess  view;  C,  author's  view     .      124 

55  Contrast  of  opposing  views  concerning  the   origin  of  arcuate 

mountains 126 

56  Mountain  arcs  of  the  northern  hemisphere  formed  at  the  close 

of  the  Paleozoic  era 127 

57  Map  of  the  arcs  of  the  eastern  United  States  with  geological 

sections 128 

58  Geological  section  across  the  eastern  base  of  the  Andes  in  north- 

western Argentina  (after  Palmer) 129 

59  Geological  sections  of  the  Rocky   Mts.  facing  the   Laramide 

Sea 130 

60  Map  of  the  Cretaceous  Ocean  and  the  Rocky  Mts.  which  rose 

about  its  border 131 

61  Arcs  of  the  Pacific  Coast  of  the  United  States  as  indicated  by 

strike  directions 132 

62  Sketch-map  of  the  great  depression  formed  in  Pleistocene  time, 

between  the  Rocky  Mts.  and  the  Pacific  Coast  of  the  United 
States,  together  with  sections 133 

63  Contrasted  views  of  Suess  and  Hobbs  concerning  the  origin  of 

the  mountain  arcs  of  the  Western  United  States     ....     134 

64  Plan  of  an  arc  rising  in  a  reentrant  of  the  coast 136 

65  Plan  of  an  arc  rising  off  an  obtuse  salient  of  the  coast    .     .     .     136 

66  Plans  of  arcs  in  two  successive  stages  formed  off  an  acute  salient 

of  the  coast 137 

67  Sections  of  rising  arcs 138 

68  Sketch-map  of  the  arcs  formed  off  the  eastern  front  of  the 

Rocky  Mts.  (based  on  map  by  U.  S.  Geological  Survey)     .     140 

69  Series   of   sections   through   the   Bighorn   Range   showing   in- 

ward underthrust  in  the  center  combined  with  inward  over- 
thrust  on  the  flanks   (Darton's  sections) 141 

70  Series  of  sections  showing  the  partly  buried  arc  of  the  Black 

Hills  with  its  inward  underthrust  in  the   center  combined 
with  inward  overthrust  on  the  flanks  (Darton's  sections)      .      142 

71  Schematic  diagram  to  show  plan  of  formation  of  rising  arc  at 

the  front  of  a  lense  of  sediments 143 

72  Sketch  map  to  show  the  arcs  of  southeastern  Asia     ....     147 

73  Virgation  of  the  arcs  of  the  Andes  at  the  Peruvian  knots  (after 

Karsten'a  geological  map) 148 


LIST  OF  ILLUSTRATIONS  xvii 


FIGURE 

71  Samples  of  the  pattern  of  the  fracture  network  of  the  earth. 
(A),  the  network  itself;  1,  rectangular  system  of  master  joints; 
3,  escarpment  due  to  master  joints  in  sub-equally  spaced 
series  near  Cayuga  Lake,  N.  Y.;  4,  joint  system  with  com- 
posite groups  and  a  patterned  fault  system  evolved  from  the 
joint  system  by  displacement,  Norwegian  coast;  9,  ground 
plan  of  the  system  with  pattern  changing  its  position;  10, 
fractures  produced  in  block  of  an  elastic  substance  (moulder's 
wax)  by  compression  from  the  ends.  (B),  Imprint  of  the 
fracture  pattern  in  the  earth's  face;  2,  patterned  drainage 
lines  of  an  area  in  Connecticut;  5,  the  gigantic  fault-rifts  of 
East  Central  Africa;  6,  the  fracture  controlled  course  of  a 
canyon  in  Swedish  Lapland;  7,  "Checker-board"  drainage  of 
Western  Ontario;  8,  map  of  the  Batoka  Gorge  below  the 
Victoria  Falls,  Rhodesia,  controlled  by  the  fracture  network; 
11,  drainage  of  the  "dolomites"  of  the  Tyrol  controlled  by 
the  fracture  pattern  ......  '.  ......  154 

75  Map  showing  the  fracture  network  of  the  Island  of  Celebes  in 

its  relation  to  the  fold  lines  (after  Ahlburg)      .....     155 

76  Sketch-map  showing  the  prevailing  direction  of  joint  fissures  in 

Southern  South  America  (after  Windhausen)      .....      156 

77  Schematic  diagram  to  illustrate  the  relation  of  the  fractures  to 

the  growing  anticline     .............     157 

78  Comparison  of  the  average  composition  of  Pacific,  Predazzic 

and  Atlantic  rock  types  with  the'  average  igneous  rock     .     .     161 

79  Comparative    composition    of    andesitic,    basaltic    and    feld- 

spathoidal   lavas       ..............      162 

80  Section  across  the  arc  of  the  New  Hebrides  (after  Mawson)     .      163 

81  Map  of  the  world  on  Mollweide  projection  to  show  the  dis- 

tribution of  andesitic  and  non-andesitic  lava  types     .     .     .     165 

82  Idealized  successive  sections  to  illustrate  splitting  of  original 

Andesitic  volcanic  magmas  into  basaltic  or  feldspathoid  types    166 

83  Diagrams    to    illustrate    the    average    composition    of    arcuate 

mountain  (fold)  lava  and  fault-block  lava  in  comparison  with 

the  average  magma      .     .......     .....      171 

84  Diagram  to  illustrate  a  manner  of  formation  of  magma  cham- 

bers by  block  faulting  .............     172 


EARTH  EVOLUTION 

AND  ITS 

FACIAL  EXPRESSION 


EARTH  EVOLUTION  AND  ITS 
FACIAL  EXPRESSION 

CHAPTER  I 

THE  NEBULAR  HYPOTHESIS  AND  THE  SUPPOSED 
EARTH  CRUST 

THE  early  belief  that  volcanic  lava  is  the  material  com- 
posing a  fused  earth  interior  is  to  be  ascribed  to  the  frequent 
exudation  of  lava  from  the  active  vents  distributed  about  the 
Mediterranean  region,  within  which  region  our  civilization 
developed.  The  continuation  of  this  doctrine  to  the  philoso- 
phies of  medieval  and  modern  times  is  not  difficult  to  under- 
stand. It  was  but  natural  that  primitive  peoples  face  to  face 
with  the  awe-inspiring  phenomena  of  a  volcanic  eruption 
should  assume  that  the  liquid  lava  is  issuing  through  an 
opening  in  a  solid  crust  above  a  liquid  reservoir. 

As  early  as  611  B.C.  Anaximander  assumed  the  earth  and 
the  stars  to  be  made  from  liquid  material,  and  Heraclitus  a 
century  later  made  fire  the  original  element.  About  the 
same  time  Pythagoras  held  the  earth  to  be  a  sphere  com- 
pleting a  rotation  about  central  fires  once  every  twenty-four 
hours.  Near  the  beginning  of  the  Christian  era  the  geogra- 
pher Strabo  accounted  for  the  islands  which  suddenly  ap- 
peared in  the  Mediterranean  as  raised  up  by  internal  fires. 
Seneca  made  volcanoes  the  definite  canals  which  connect  the 
internal  fires  with  the  surface  of  the  earth. 

The  supposed  circumscribed  limits  of  the  early  world  gave 
necessarily  an  idea  of  universality  to  the  doctrine  of  internal 
heat  for  the  earth,  and  with  the  extension  of  geographical 

1 


2          EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

exploration  the  discovery  of  many  new  active  volcanic  vents 
served  only  to  confirm  the  earlier  impression. 

The  lands  bordering  upon  the  Mediterranean  are  even 
more  in  the  realm  of  earthquakes  than  they  are  of  volcanoes, 
and  at  the  time  of  all  heavy  earthquakes  water  spurts  up 
from  the  ground  under  pressure  to  produce  fountains,  much 
as  would  the  water  which  enters  the  hold  of  a  ship  through 
an  opened  seam.  It  is  probably  this  which  largely  explains 
the  Babylonian  conception  of  waters  below  as  well  as  above 
the  earth,  with  the  earth  floating  as  a  disk  upon  the  waters 
below.  This  view,  taken  over  by  the  Hebrews,  seemed 
naturally  to  be  confirmed  when  their  limited  territory  was 
found  to  be  extended  outward  by  the  seas.  The  conception 
of  bodies  of  water  within  the  earth's  crust  was  a  rather 
general  one  up  to  the  time  of  Descartes. 

The  Cartesian  philosophy  which  forms  the  basis  of  our 
modern  doctrines,  contains  in  it  the  germ  of  the  Nebular 
Hypothesis,  which  was  later  developed  by  Kant,  Laplace, 
Herschel,  and  others — an  hypothesis  which  is  primarily  re- 
sponsible for  fastening  upon  us  not  only  a  false  conception  of 
the  nature  of  our  planet,  but  an  erroneous  notion  of  the 
origin  of  lava  as  well.  Descartes7  contribution  to  the  Nebu- 
lar Hypothesis  was  his  idea  of  tourbillons  or  vortices  in 
nebulous  matter  about  the  sun  and  stars,  and  in  his  concep- 
tion of  the  earth  planet  he  combined  the  ideas  which  had 
grown  up  from  the  observation  of  both  earthquakes  and  vol- 
canoes. To  a  core  of  glowing  hot  matter  he  added  local 
substrata  of  water  which  he  believed  to  be  a  result  of  infall 
attended  with  local  elevation  of  the  earth's  crust  which  was 
composed  of  rock.  These  water  substrata  thus  naturally 
came  to  be  in  the  neighborhood  of  the  mountain  ranges, 
within  which  neighborhood  the  earthquake  fountains  are 
seen. 

The  three  types  of  matter  according  to  Descartes,  who 
flourished  from  1596  to  1650,  were  the  glowing  and  hot,  the 
transparent,  and  the  opaque  and  cold ;  and  in  his  discussion 


THE  SUPPOSED  EARTH  CRUST  3 

of  their  relationships  the  idea  of  a  surface  crust  above  molten 
nuclei  is  clearly  brought  out.  The  opaque  sun  spots  through 
enlargement  and  coming  together  could  form  a  crust  which 
would  extinguish  the  glowing  orb,  as  he  believed  had  already 
occurred  in  the  case  of  the  earth;  whereas  by  diminishing 
they  could  increase  the  sum  of  the  sun's  luminosity.  In  his 
view  the  extinguished  earth  had  lost  its  own  vortex  and  been 
drawn  into  that  of  the  sun. 

The  great  mathematician  Leibnitz  (1646-1716),  who  fol- 
lowed Descartes,  like  him  conceived  the  earth  to  have  been  a 
glowing  mass  that  had  gone  out  for  lack  of  further  com- 
bustible materials,  leaving  a  "glass-like"  crust  enveloped  in 
an  original  universal  ocean  derived  from  condensation  of  the 
enveloping  vapors,  and  by  an  outer  envelope  of  air. 

Swedenborg  (1688-1772)  developed  the  Cartesian  vortices 
somewhat  further  in  the  direction  of  the  later  Nebular  Hy- 
pothesis, through  making  them  break  up  by  withdrawal  of 
matter  from  the  poles  into  an  equatorial  ring,  within  which 
ring  planets  and  satellites  developed. 

In  the  latter  half  of  the  eighteenth  century  was  reached 
the  culminating  period  for  theories  of  the  universe.  The 
Nebular  Hypothesis  now  took  on  the  definite  form  which  it 
has  held  almost  to  the  present  day.  Quite  independently,  it 
would  seem,  a  great  philosopher,  Kant;  a  great  mathe- 
matical physicist,  Laplace;  and  an  astronomer-observer,  Sir 
William  Herschel,  arrived  at  similar,  even  if  not  identical, 
conceptions  of  the  universe.  Kant,  the  pioneer  of  the  group, 
built  his  theory  upon  the  framework  of  the  Cartesian  con- 
ception, adopting  also  some  ideas  from  the  atomistic  school 
of  Greek  philosophers,  more  especially  Democritus  and 
Lucretius,  the  latter  his  favorite  classical  author.  To  his 
great  contemporary  Newton,  who  it  should  be  remembered 
conceived  of  the  earth  as  having  a  molten  interior,  Kant 
owed  much ;  but  he  built  his  theory  largely  upon  analogy, 
going  out  from  the  equatorial  rings  of  the  planet  Saturn. 
We  have  here  one  of  those  amazing  instances  so  common 


4          EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

in  the  evolution  of  thought,  in  which  the  altogether  ex- 
ceptional rather  than  the  common  has  been  taken  as  a 
type.  To  the  uniformity  of  direction  of  motion  of  the 
planets  and  their  satellites  within  their  orbits,  at  the  time 
believed  to  be  universal,  Kant  added  as  the  basis  of  his 
theory  important  contributions  of  his  own — he  clearly  set 
forth  the  necessary  diminution  in  the  earth's  rotational 
velocity  because  of  the  friction  of  tidal  currents,  and  in 
1765  he  introduced  the  idea  that  the  contraction  of  the 
sun's  mass  must  develop  a  high  temperature  within  it. 

The  development  of  his  theory  early  in  Kant's  career 
and  at  a  time  when  he  had  not  become  known  as  a  great 
philosopher,  explains  why  its  publication  brought  compara- 
tively little  response.  Eight  years  later,  in  1763,  he  in- 
troduced into  his  philosophy  a  summary  of  the  hypothesis 
under  the  title,  "The  Only  Possible  Argument  for  a  Dem- 
onstration of  the  Existence  of  God,"  in  which  summary  is 
to  be  found  the  best  outline  of  his  theory.  He  conceived 
of  discreet  and  cold  solid  particles  scattered  uniformly  in 
space,  which  particles  under  the  action  of  the  law  of  gravi- 
tation came  together  about  centers  or  nuclei  to  produce 
rotating  vortices.  These  upon  further  condensation  flat- 
tened about  the  poles  of  their  rotating  axes  as  the  velocity 
of  rotation  increased,  and  with  further  acceleration  of  the 
velocity,  equatorial  rings — like  those  of  Saturn — were 
thrown  off.  From  these  in  turn  planets  were  formed  which 
rotated  within  orbits  which  had  the  positions  of  the  earlier 
rings. 

The  great  French  savant  Laplace  developed  the  Nebular 
Hypothesis  independently  and  much  more  definitely  than 
Kant  had  done.  Instead  of  being  a  philosopher  he  was  an 
astronomer  and  physicist,  and  was  rated  as  the  greatest  of 
all  scientists  after  Newton.  Yet,  curiously  enough,  the 
famous  Laplacian  theory  of  the  origin  of  the  universe,  which 
for  more  than  a  century  has  been  a  foundation  stone  of 
modern  science,  was  apparently  not  a  conception  carefully 


THE  SUPPOSED  EARTH  CRUST  5 

elaborated  and  rigorously  tested,  as  were  his  other  studies, 
but  it  was  thrown  off  as  an  undigested  afterthought  which 
might  perhaps  be  important.  In  the  fourteen  massive 
tomes  which  make  up  Laplace's  complete  works,  his  nebular 
hypothesis  takes  up  only  ten  pages  of  a  note — "Note  VII 
and  Last" — appended  to  the  volume  treating  the  Exposition 
du  Systeme  du  Monde. 

Looking  back  over  the  intervening  century  in  the  light 
of  the  recent  studies  which  have  convincingly  proven  the 
fallacies  of  the  Laplacian  conception,  we  are  able  to  see 
how  its  success  is  to  be  largely  ascribed  to  the  fame  of  the 
author  of  the  Mecanique  Celeste,  a  work  never  rivaled  in 
its  field,  of  which  indeed  it  has  been  said  that  any  one  of 
its  twenty-four  parts  would  have  made  the  reputation  of 
a  man  of  science.  It  is  this  eminence  of  Laplace  as  a  sci- 
entist which  has  carried  his  theory  to  general  acceptance 
quite  regardless  of  its  inherent  inadequacy. 

Unlike  Kant,  who  thought  of  the  original  world  stuff  as 
discrete  solid  particles,  really  meteoric  matter,  Laplace  con- 
ceived space  to  be  filled  with  a  highly  heated  and  extremely 
tenuous,  gaseous  nebula;  though  it  may  be  doubted  if  he 
knew  of  the  actual  existence  of  such  matter,  which  his  con- 
temporary, Sir  William  Herschel,  was  the  first  to  describe. 
This  hot  gaseous  matter  in  Laplace's  conception  extended 
beyond  the  orbit  of  the  most  remote  planet  and  had  the 
form  of  a  rotating  spheroid,  its  intense  heat  being  account- 
able for  the  high  rarefaction.  Loss  of  heat  through  radia- 
tion into  space  brought  about  continuing  refrigeration  and 
resultant  contraction  with  the  inevitable  effect  of  an  ac- 
celerated rotational  velocity.  This  ever  increasing  velocity 
produced  centrifugal  effects  which  reduced  the  polar  or 
axial  diameter  of  the  gaseous  spheroid  at  the  same  time 
that  it  produced  swelling  within  the  equatorial  zone. 
Eventually  this  centrifugal  force  would  become  equal  to 
the  centripetal  force  of  gravitation  for  those  portions  of 
the  gaseous  mass  which  were  most  remote  from  the  center, 


6          EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

a*id  the  outer  equatorial  portion  of  the  spheroid  could  in 
consequence  contract  no  further.  The  concentrated  force 
of  gravitation  being  however  for  any  portion  of  the  mass 
inversely  as  the  square  of  the  distance  from  the  center,  the 
central  core  would  continue  to  shrink  and  so  draw  away 
from  the  outer  equatorial  belt.  This  belt  would  therefore 
separate  from  the  parent  mass  as  an  equatorial  ring.  Here 
the  analogy  with  the  unique  rings  of  Saturn  is  apparent. 

With  continuation  of  the  refrigerating  process,  other 
rings  would  separate  in  turn,  all  subject  to  rotation  in  the 
same  sense  as  the  residual  spheroidal  nucleus.  Irregularity 
of  distribution  of  matter  within  each  ring  would  in  tune 
cause  a  drawing  together  within  the  ring  about  some  cen- 
ter so  as  to  form  a  rotating  spheroid  or  planet  whose  trans- 
lational  motion  would  be  within  the  space  of  the  former 
ring  as  an  orbit,  and  would  have  its  own  rotational  velocity 
in  the  same  clockwise  (or  counter-clockwise)  direction  as 
that  of  the  orbital  movement.  Further  refrigeration  would 
result  in  the  formation  of  rings  and  later  spheroids  (satel- 
lites), or  rings  only  as  in  Saturn,  in  the  rare  event  of  per- 
fect regularity  of  distribution  of  density  of  the  nebulous 
matter. 

Thus  far  the  Laplacian  theory  provides  for  systems  of 
spheroids  of  gaseous  matter  having  relative  positions  and 
orbital  and  rotational  motions  in  general  similar  to  those 
of  our  solar  system.  But  the  earth  and  its  sister  planets 
being  solids,  at  least  at  the  surface,  it  is  necessary  to  carry 
the  process  farther.  Continued  refrigeration  is  invoked  to 
produce,  first,  nuclei  of  liquid  spheroids  surrounded  by  a 
vaporous  envelope;  and,  still  later  the  refrigeration  is  fur- 
ther called  upon  to  accomplish  the  next  transformation, 
namely,  that  from  a  liquid  to  a  solid  crust — from  molten 
magma  to  solid  rock  such  as  we  know  under  the  term 
igneous  rock. 

It  is  at  this  point  that  a  vitally  important  step,  but  one 
which  has  been  little  considered,  has  been  taken  in  devel- 


THE  SUPPOSED  EARTH  CRUST  7 

oping  the  conception.  At  the  time  Laplace's  hypothesis 
was  promulgated  it  was  not  known  whether  solid  igneous 
rock  or  its  liquid  equivalent  possessed  the  greater  density. 
That  discovery  is  a  very  recent  one,  and  in  the  absence  of 
exact  knowledge  it  is  quite  evident  that  analogy  determined 
the  form  which  the  hypothesis  was  to  take.  This  analogy 
was  to  be  made,  not  with  a  general  but  with  an  exceptional 
case,  as  had  already  been  true  of  the  misleading  analogy 
with  Saturn's  rings  in  the  matter  of  the  separated  equatorial 
sections  of  the  nebula.  It  is  a  fact  fuU  of  significance  that 
Descartes,  Newton,  Kant,  Laplace  and  Herschel,  all  of 
whom  played  a  part  in  the  building  of  this  "Grandest  Con- 
ception of  the  Universe,"  were  residents  of  northern  Europe 
where  water  is  in  the  winter  season  habitually  seen  to  con- 
geal into  a  surface  crust,  and  they  all  alike  held  the  view 
that  the  earth  had  a  crust  of  solid  rock  which  floated  upon 
a  central  core  of  its  liquid  equivalent.  Of  bodies  known  in 
both  the  solid  and  liquid  conditions,  water  is  the  one  con- 
cerning which  it  was  definitely  known  that  the  density  in 
the  solid  form  was  less  than  in  the  liquid.  This  common 
and  well  known  substance  whose  transformation  from  one 
state  of  aggregation  to  the  other  had  so  often  been  ob- 
served that  the  knowledge  had  come  to  be  an  unconscious 
acquisition,  thus  had  with  them  all  the  force  of  an  axiom. 
Had  the  Nebular  Hypothesis  been  put  forth  by  men  who 
had  long  been  residents  of  a  tropical  or  sub-tropical  coun- 
try, it  is  extremely  unlikely  that  this  fatal  slip  would  have 
been  made,  and  it  is  certain  that  science  would  in  that 
event  have  enormously  profited.  Of  this  vitally  important 
subject  we  shall  have  more  to  say  in  the  next  chapter.  It 
is  pertinent  now  to  consider  especially  the  ways  in  which 
the  hypothesis  as  a  whole  has  been  found  wanting  and 
is  being  abandoned  for  another  conception.  This  though 
still  in  the  realm  of  hypothesis  rather  than  theory,  is  at 
least  not  open  to  the  serious  objections  to  the  Laplacian 
form  of  the  Nebular  Hypothesis,  so  long  accepted  on  the 


8          EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

prestige  of  its  sponsor  without  either  serious  questioning 
or  scientific  testing. 

It  is  hardly  necessary  for  our  purpose  to  here  supply 
the  details  of  the  various  modifications  which  the  Nebular 
Hypothesis  has  since  undergone  in  the  hands  of  Sir  William 
Herschel,  Sir  George  Darwin,  and  Sir  Norman  Lockyer. 
All  of  them  alike  followed  Kant  rather  than  Laplace  in 
conceiving  the  cosmic  matter  out  of  which  the  system  of  the 
universe  has  been  made,  to  be  in  the  main  not  gaseous, 
but  discrete  solid  particles.  It  cannot  be  claimed,  how- 
ever, that  any  of  them  in  any  important  way  threw  dis- 
credit upon  the  idea  of  a  once  molten  interior  of  the  earth. 

It  was  Chamberlin's  recent  and  elaborate  studies  in 
association  with  the  astronomer  Moulton  which  first 
brought  into  clear  relief  the  fallacies  of  a  hypothesis  which 
had  been  supported  as  much  for  the  grandeur  of  its  con- 
ception as  by  reason  of  the  enormous  prestige  of  its  author. 
First  of  all,  it  was  to  be  noted  that  observational  astronomy 
has  shown  that  the  dominant  type  of  nebula  is  not  a  ring 
but  a  spiral,  the  analogy  of  the  rings  of  Saturn  having  been 
responsible  for  this  fundamental  error  in  the  Nebular 
Hypothesis.  Only  a  few  ring  nebulae  are  known,  whereas 
120,000  spiral  nebulae  have  already  been  observed,  and  the 
total  number  must  be  very  much  greater.  Other  considera- 
tions are  no  less  cogent  as  objections  to  the  Kant-Laplace 
hypothesis. 

It  is  a  firmly  established  law  of  mechanics  that  the  mo- 
ment of  momentum  of  any  freely  rotating  or  revolving 
system,  when  not  influenced  by  outside  forces,  remains 
constant  without  regard  to  changes  which  may  go  on  within 
the  system.  Starting  from  this  premise,  Moulton  has 
shown  that  if  the  solar  system  were  converted  into  a  gaseous 
spheroid  large  enough  to  fill  all  the  space  within  the  orbit 
of  the  most  distant  planet,  and  if  this  gaseous  matter  were 
distributed  in  density  as  is  required  by  the  known  laws  of 
gases;  and  if,  further,  the  system  were  given  the  moment 


THE  SUPPOSED  EARTH  CRUST  9 

of  momentum  now  possessed  by  the  solar  system;  the 
gaseous  spheroid  would  not  have  a  velocity  of  rotation  suffi- 
cient to  detach  a  ring  of  matter  from  its  equator,  as  is 
required  by  the  Kant-Laplace  hypothesis;  and,  moreover, 
it  could  not  acquire  such  a  velocity  until  it  had  contracted 
well  within  the  orbit  of  Mercury  and  close  to  the  sun.  To 
have  detached  the  ring  from  which  Neptune  is  conceived  to 
have  evolved  would  have  required  a  moment  of  momentum 
200  times  greater  than  it  now  possesses,  and  to  have  de- 
tached the  earth  ring  a  moment  of  momentum  1800  times 
as  great  as  that  available  would  have  been  required. 

No  less  striking  discrepancies  are  discovered  when  the 
masses  of  the  planets  are  subjected  to  scrutiny.  The  mass 
of  the  ring  from  which  Jupiter  and  its  satellites  were  as- 
sumed to  have  been  formed,  must  have  been  less  than  one- 
tenth  of  one  percent  of  the  original  nebula,  though  it  must 
have  carried  off  ninety-five  percent  of  the  total  mo- 
ment of  momentum.  Together,  the  planets  with  their 
satellites  contain  about  one  seven-hundredth  of  the  matter 
of  the  system,  though  they  possess  more  than  ninety-seven 
percent  of  its  moment  of  momentum.  The  inner  planets 
should,  moreover,  possess  the  greater  masses,  but  this  is 
not  the  case.  The  sun  should  have  a  great  velocity  with 
a  corresponding  eccentricity.  As  a  matter  of  fact,  it  rotates 
very  slowly,  and  its  eccentricity  is  inappreciable.  Further- 
more, its  plane  of  rotation  varies  by  seven  degrees  from 
that  of  the  average  rotation  plane  of  its  system. 

The  planetesimal  hypothesis  which  Chamberlin  has  after 
long  years  of  study  put  forward  to  replace  the  Kant- Laplace 
hypothesis  of  the  universe,  conceives  the  material  from 
which  the  system  developed  to  be  innumerable  bodies,  each 
having  the  magnitude  of  sand  or  dust,  and  that  these  re- 
volve about  a  central  gaseous  mass  as  the  planets  do  today. 
As  far  back  as  the  hypothesis  carries  us,  this  material  was 
conceived  to  be  kept  dispersed  by  the  centrifugal  accelera- 
tion due  to  the  rotation  of  the  system.  The  evolution  of 


10        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

the  solar  system  has  been  a  process  of  aggregation  or  as- 
semblage of  scattered  material.  Gases,  while  not  excluded 
from  the  hypothesis,  play  but  a  subordinate  role.  This 
hypothesis  is  today  the  one  which  has  the  most  general 
support  among  students  of  earth  science,  and  the  Kant- 
Laplace  hypothesis  is  already  relegated  to  the  category  of 
schemes  which,  under  the  rigid  test  of  searching  investiga- 
tions, have  been  discredited  and  abandoned. 

For  our  purpose  it  is  chiefly  of  interest  to  note  that  the 
planetesimal  hypothesis,  unlike  its  predecessors,  does  not 
call  for  an  originally  molten  planet  and  thus  provide  a 
ready-made  source  of  volcanic  materials — lava,  gases,  and 
vapors. 

It  has  been  necessary  to  go  over  this  somewhat  ex- 
tended treatment  of  the  field  of  cosmogeny  in  order  to 
clear  the  decks,  as  it  were,  for  an  untrammeled  scientific 
discussion  of  the  development  of  continents  and  mountains. 
Our  concern  has  been  chiefly  with  those  sections  of  the 
theories  of  the  universe  which  relate  to  the  earth  planet, 
but  students  who  are  especially  interested  in  the  larger 
problems  of  the  origin  of  the  sun  and  stars,  should  not  fail 
to  review  the  Harvard  studies  upon  the  age  of  the  stars, 
and  the  conclusions  which  Professor  Campbell  has  formed 
from  them.  We  are  yet  far  from  a  complete  solution  of 
this  grandest  of  all  undiscovered  secrets,  the  origin  of  the 
universe,  but  the  efforts  to  penetrate  beyond  the  veil  will  go 
on  unceasingly.  Galileo  said  truly:  "There  are  such  pro- 
found secrets  and  such  lofty  conceptions  that  the  night 
labors  and  the  researches  of  hundreds  and  yet  hundreds  of 
the  keenest  minds,  in  investigations  extending  over  thou- 
sands of  years,  would  not  penetrate  them,  and  the  delight 
of  searching  and  finding  endures  forever." 

LITERATURE 

KARL  VON  ZITTOL.  History  of  geology  and  paleontology  to  the  end  of  the 
19th  century,  translated  by  M.  Ogilvie-Gordon,  London,  1901,  intro- 
duction and  chapter  I. 


THE  SUPPOSED  EARTH  CRUST  11 

IMMANUEL  KANT.  Gesammelte  Schriften,  vol.  1,  and  vol.  2,  div.  II,  sec.  7 
(Cosmogonie). 

WILLIAM  HASTIE.    Kant  cosmogeny,  etc.    Glasgow,  1900,  p.  205. 

PIERRE  SIMON  LAPLACE.  Exposition  du  Systeme  du  Monde,  complete  works, 
6th  (French)  edition,  vol.  6,  Note  VII  and  Last,  pp.  498-509. 

SVANTE  ARRHENIUS.  Die  Vorstellung  vom  Weltgebaiide  im  Wandel  der 
Zeiten,  Das  Werden  der  Welten,  Neue  Folge,  Leipzig,  1900,  p.  191. 

SIR  ROBERT  S.  BALL.  The  earth's  beginning,  Appleton,  New  York,  1902, 
p.  384. 

T.  C.  CHAMBERLIN  and  R.  D.  SALISBURY.  Geology,  Holt,  New  York,  1906, 
vol.  2,  chaps.  I-II. 

THOMAS  CHROWDER  CHAMBERLIN.  The  Origin  of  the  Earth,  University  of 
Chicago  Press,  1916,  p.  271. 

W.  W.  CAMPBELL.  The  evolution  of  the  stars  and  the  formation  of  the 
earth,  Scientific  Monthly,  vol.  1,  1915,  pp.  1-17,  177-194,  238-255. 

JOSEPH  BARRELL.  The  origin  of  the  earth,  chapter  I  of  Lull's,  The  evolution 
of  the  earth  and  its  inhabitants,  Yale  University  press,  1918,  pp.  1-44. 

T.  C.  CHAMBERLIN.  An  attempt  to  test  the  nebular  hypothesis  by  the  re- 
lations of  masses  and  momenta,  Jour.  Geol.,  vol.  8,  1900,  pp.  58-73. 

T.  C.  CHAMBERLIN.  Diastrophism  and  the  formative  processes,  XI,  Se- 
lective segregation  of  material  in  the  formation  of  the  earth  and  its 
neighbors,  ibid.,  vol.  28,  1920,  pp.  126-157. 


CHAPTER  II 

THE  NATURE  OF  THE  EARTH'S  INTERIOR 

ABANDONMENT  of  the  Kant-Laplace  hypothesis  has,  as 
we  have  seen,  brought  release  from  an  error  fundamental 
in  our  science.  The  almost  blind  acceptance  of  its  require- 
ments reacted  as  much  upon  the  advance  of  geological  sci- 
ence as  it  did  upon  astronomy.  Now  released  from  the 
thrall  of  a  scientific  dogma,  geologists  are  no  longer  under 
the  obligation  to  regard  volcanic  lava  as  derived  from  a 
fluid  earth  interior. 

While  no  such  blind  adherence  to  the  planetesimal  hy- 
pothesis is  to  be  observed,  that  hypothesis  has  been  tested 
insofar  as  it  is  possible  to  apply  tests.  It  is  of  considerable 
interest  to  examine  its  requirements  in  respect  to  the  quan- 
tity of  heat  which  should  be  developed  in  the  growing  earth 
and  the  consequent  physical  condition  of  the  planet's 
interior  portion. 

The  theory  requires  that  heat  be  developed  both  from 
impact  of  the  discrete  bodies  which  arrive  at  the  earth's 
surface,  and  from  increasing  compression  under  the  action 
of  gravity  so  soon  as  the  nucleus  has  attained  considerable 
proportions.  The  discrete  particles  are  conceived  to  be 
moving  in  space  within  elliptical  orbits  and  with  orbital 
velocities  measured  in  the  same  clockwise  direction,  and, 
further,  with  these  orbital  velocities  varying  in  different 
parts  of  the  orbit.  Collisions  can  occur  only  through  one 
body  overtaking  another  whenever  orbits  become  tangent. 
The  effective  velocity  of  impact  must  be,  therefore,  not  the 
sum,  but  the  difference,  of  the  velocities  of  the  two  bodies 
on  impact,  and  at  most  but  a  few  miles  per  second. 

At  the  present  time  the  earth  planet  is  shrinking  faster 

12 


THE  NATURE  OF  THE  EARTH'S  INTERIOR       13 

than  it  grows,  and  though  an  estimated  twenty  million 
meteoric  bodies  reach  its  surface  from  space  during  each 
year,  no  less  than  a  billion  years  would  be  required  to  in- 
crease the  earth's  diameter  from  this  source  by  a  single  inch. 
If  we  assume  that  one  hundred  tons  of  meteoric  matter 
reach  the  earth's  surface  daily  at  an  average  velocity  rela- 
tive to  the  earth  which  may  be  even  as  high  as  twenty 
miles  per  second,  this  material  could  supply  in  a  year  only 
as  much  heat  as  the  sun  is  giving  us  in  one- tenth  of  a 
second.  These  values  can,  however,  be  but  a  fraction  of 
those  which  were  characteristic  of  the  planet  during  its 
early  stages  of  relatively  rapid  growth  from  the  infall  of 
meteoric  bodies. 

A  fact  the  significance  of  which  seems  to  have  been  gen- 
erally overlooked  by  students  of  the  subject,  is  that  the 
meteoric  material  which  now  comes  to  the  earth  is  almost 
exclusively  the  less  dense  stony  meteorite,  whereas  the 
density  of  the  earth  as  a  whole  shows  clearly  that  the 
greater  portion  of  the  centrosphere,  or  inner  core  of  the 
earth,  must  be  made  up  of  the  dense  nickel-iron  meteoric 
matter.  The  density  of  the  earth  as  a  whole  as  determined 
by  astronomers  is  5.6,  and  the  average  density  of  the  known 
meteorites,  predominantly  irons,  is  about  the  same,  that 
of  the  analyzed  meteorites  being  5.57.  The  meteoric  stones, 
on  the  other  hand,  have  an  average  density  of  about  3.6, 
which  is  somewhat  higher  than  that  of  the  earth's  outer- 
most shell,  which  is  usually  given  as  2.67.  This  outermost 
shell  of  the  earth,  unlike  any  lower  ones,  as  we  believe, 
contains  a  considerable  proportion  of  sediments — shales, 
sandstones  and  limestones — which  show  the  reaction  with 
the  earth's  atmosphere.  The  proportions  of  shale,  sandstone 
and  limestone  have  been  estimated  by  Clarke  to  be  re- 
spectively as  19,  3,  and  1,  and  the  corresponding  densities 
may  be  assumed  to  be  2.6,  2.41,  and  2.7.  With  the  two 
former,  quartz  or  silica  is  the  dominating  constituent,  with 
a  specific  gravity  of  2.56.  Could  we  secure  an  average  for 


14        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

the  shell  underlying  the  sedimentary  strata — probably  less 
than  ten  kilometers  in  thickness — it  would  undoubtedly  be 
considerably  heavier  and  probably  not  far  from  the  average 
value  3.6  determined  for  meteoric  stones. 

Nothing  that  we  know  concerning  the  compressibility  of 
rock  materials,  and  especially  that  of  the  metals,  would  war- 
rant us  in  thinking  that  the  large  margin  of  density  of  the 
earth  as  a  whole  above  that  of  its  surface  portion,  can  be 
accounted  for  through  compression.  The  metals  are  re- 
garded as  almost  incompressible,  and  the  known  density  of 
the  meteoric  irons  and  stone-irons  makes  it  easy  to  account 
for  earth  density  without  recourse  to  compression.  If,  in 
addition,  we  have  in  mind  the  revelations  of  the  spectro- 
scope concerning  the  unity  of  the  visible  universe  as  regards 
constituent  chemical  materials,  and,  further,  that  of  all 
the  chemical  elements  known  iron,  nickel  and  cobalt  are  the 
only  ones  which  possess  magnetic  properties;  the  strong 
earth  magnetism  is  compelling  in  forcing  us  to  the  conclu- 
sion that  the  earth's  interior  is,  like  that  of  its  neighboring 
meteoric  bodies,  in  large  part  composed  of  these  three  ele- 
ments as  they  are  found  associated  in  meteoric  irons. 

Students  of  earth  physics  have  been  put  to  great  diffi- 
culties to  explain  this  arrangement  of  denser  and  lighter 
matter  within  the  earth  without  at  the  same  tune  assuming 
that  the  whole  mass  was  once  molten  and  subject  to  the 
convectional  currents  due  to  differences  of  gravity  within 
liquids.  Chamberlin's  explanation,  which  assumes  the  ex- 
istence of  radial  threads  of  molten  material  throughout  the 
mass,  is  not  easy  to  accept.  To  the  writer  it  has  seemed 
that  an  explanation  may  be  looked  for  in  a  wholly  dif- 
ferent direction. 

It  seems  generally  to  have  been  overlooked,  at  least  so 
far  as  its  significance  is  concerned,  that  of  the  total  of  about 
350  known  "falls''  of  meteorites,  all  but  ten  have  been  of 
meteoric  stone.  Though  only  ten  meteoric  irons  have  been 
seen  to  fall  and  been  collected,  yet  the  larger  number  of 


THE  NATURE  OF  THE  EARTH'S  INTERIOR       15 

specimens  gathered  in  museums  are  of  meteoric  irons.  This 
is,  of  course,  for  the  reason  that  meteoric  stones  generally 
bear  such  close  resemblance  to  our  terrestrial  rocks,  as  to 
attract  little  attention  and  to  be  distinguishable  only  by 
the  expert.  They  are  hence  little  likely  to  be  discovered. 
Meteoric  irons,  on  the  other  hand,  are  unusual  in  appear- 
ance, are  heavy  and  with  somewhat  remarkable  surface  fea- 
tures which  attract  attention,  and  they  moreover  are  apt 
to  arouse  hopes  of  finding  valuable  ores  of  iron  in  the 
vicinity.  They  are  thus  likely  to  be  collected  and  submitted 
to  examination  by  experts.  Their  predominance  in 
meteorite  collections  has  largely  obscured  the  fact  that 
they  have  seldom  been  seen  to  fall,  and  we  are  entitled  to 
suppose  that  if  meteoric  stones  were  as  easy  to  discover  as 
are  meteoric  irons,  our  museums  instead  of  containing  only 
750  known  falls  and  finds  would  possess  about  94,000  of 
which  only  about  one  in  35  would  be  irons. 

The  only  conclusion  possible  to  draw  from  the  above  is 
that  in  the  earlier  stages  of  earth  history  the  reverse  condi- 
tion has  obtained,  and  that  the  heavy  meteoric  bodies  hav- 
ing already  been  swept  up  to  produce  the  nickel-iron 
interior  of  the  earth,  few  of  these,  but  only  the  lighter 
masses,  remain  dispersed  in  the  space  which  is  invaded  by 
our  planet. 

So  soon  as  we  examine  into  the  conditions  of  collision 
within  the  nebula  above  described,  having  due  regard  to 
the  fact  that  the  irons  have  on  the  average  a  density  nearly 
three  times  that  of  the  stones,  we  see  why  such  a  differen- 
tiation as  appears  to  have  taken  place  should  have  occurred. 
Collisions  can  occur  only  by  overtake,  as  already  explained, 
and  there  are  two  significant  cases.  If  an  iron  meteorite  over- 
takes a  stone  one,  it  loses  acceleration  as  it  imparts  in- 
creased acceleration  to  the  stone  by  a  much  larger  amount 
than  the  acceleration  of  an  iron  would  be  increased  through 
collision  with  a  stone  following  it.  Such  increases  of  acceler- 
ation in  favor  of  the  lighter  stones  would  be  sufficient  after 


16        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

one  or  more  collisions  to  change  the  orbits  of  the  lighter 
bodies  from  the  form  of  the  original  ellipse  to  one  of  greater 
eccentricity  or  to  that  of  the  hyperbola,  thus  bringing  about 
their  dispersal.* 

Let  v  and  v'  be  the  corresponding  velocities  after  impact  of  the  two 
bodies,  and  let  m  and  m',  and  u  and  u'  be  their  masses  and  velocities 
respectively  before  impact  occurs.  The  equations  for  finding  v  and  v' 
are: 

mv  -f  mV  =  mu  +  m'u' 

From  these  we  derive 

v'  —  v  —  e  (u  —  u') 

u  (m  —  em')  u'm'  (1  -f-  e) 


m  -f  m'  m  +  m' 

mu(l-fe)       +       u'  (m'  —  me) 


m  +  m'  m  -f  m' 

If  now,  m  be  made  equal  to  1  and  m'  equal  to  3,  the  relative  masses 

of  meteoric  stones  and  meteoric  nickel-iron,  we  obtain  from  the  above 
equations  : 

(With  e  ==  0)  v  =  v'  =  u/4  +  3u74 

,w..,            .  v  =  —  u/2  +  3u72 
(Withe= 


Now  as  u'  is  necessarily  less  than  u,  v'  is  also  less  than  u,  i.e.,  "the 
orbit  of  m',  since  it  is  elliptic  would  generally  remain  so. 

For  the  opposite  case  we  will  now  assume  that  m  equals  3  and  m'  equals 
1.  By  a  similar  process  we  now  derive: 

(With  e  =  0)  v  =  3u/4  +  u74 

rwithp      n         /v  =  u/2  +  u72 
(Withe-1)         \v'  =  3u/2-u72 

In  the  above  v'  is  greater  than  u,  therefore  the  orbit  of  m'  will  change 
from  the  ellipse  to  the  hyperbola  either  after  one  or  a  number  of 
impacts. 

The  additional  case  may  be  considered  for  the  case  where  the  bodies 
coming  into  collision  are  either  both  of  stone  or  both  of  iron.  Let  m 
equal  1  and  m'  equal  1,  we  then  derive 

(With  e  =  0)  v  =  v'  =  (u  +  u')/2 


As  a  consequence  of  the  above,  the  order  of  aggregation  of 
meteoric  matter  should  be:    a,  material  corresponding  to 

*At  my  request,  Dr.  Peter  Field,  Professor  of  Mathematics  in  the  Col- 
lege of  Engineering  of  the  University  of  Michigan,  has  supplied  the 
simple  equations  which  demonstrate  these  facts. 


THE  NATURE  OF  THE  EARTH'S  INTERIOR       17 

the  average  meteoric  matter  dispersed  in  the  space  which 
is  traversed  by  the  growing  planet;  b,  material  with  an 
increasingly  large  proportion  of  nickel-iron ;  and,  c,  meteoric 
stone  derived  from  that  more  widely  dispersed  and  travel- 
ing either  elliptic  orbits  of  high  eccentricity  or  on  hyper- 
bolic orbits  after  the  other  material  has  been  largely 
gathered  in. 

If  this  has  been  the  history  of  our  planet,  it  should  possess 
an  inner  core  of  only  moderate  density  and  elasticity,  an 
intermediate  thick  shell  of  maximum  density  (nickel-iron), 
and  an  outer  shell  of  relatively  low  density  and  elasticity, 
which  shell  is  enveloped  in  a  mere  skin  or  rind  of  relatively 
light  material  containing  a  large  proportion  of  rock  sedi- 
ments. 

But  we  may  go  somewhat  further  upon  the  basis  of  re- 
searches made  of  velocities  of  earthquake  waves  transmitted 
through  the  mass  of  the  earth.  As  early  as  1906  Dr.  R.  D. 
Oldham  brought  out  the  fact  that  when  earthquake  waves 
travel  within  a  central  core  of  the  earth  which  he  computed 
to  have  a  radius  two-fifths  that  of  the  whole,  their  observed 
rate  of  propagation  is  noticeably  slowed  down.  Eight  years 
later  much  more  reliable  data  were  published  by  Gutenberg 
covering  the  velocities  of  the  "first"  and  "second"  phases 
of  earthquake  waves  traversing  the  earth's  mass.  The 
velocities  of  transmission  for  these  harmonic  motions  as 
determined  by  Gutenberg  were  as  follows: 

Depth  in  "First"  Phase          "Second"  Phase 

Kilometers  Km.  Sec.  Km.  Sec. 

0  7.17  4.01 

1200  11.80  6.59 

1700  12.22  6.86 

f  13.29  r  7.32 

\  13.15  \  7.20 

2QOO  /  13'15  r  7'20 

\    8.50  \4.72 

6370  11.10  6.15 


18        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

Gutenberg  concludes:  "The  earth  consists,  therefore,  of 
a  core  (r  =  3500  km)  and  of  a  mantle  whose  constitution 
at  three  places  undergoes  changes,  though  not  abruptly." 
There  thus  appears  to  be  strong  evidence  that  matter  of 
a  different  composition  from  that  which  envelops  it  occupies 
this  inner  core.  Under  our  theory  of  selective  dispersion  of 
meteoric  matter  within  the  swarm  about  the  growing  earth 


FIG.  1. — Diagram  to  illustrate  a  theory  of  the  constitution  of  the  earth 

nucleus,  there  should  develop  a  core  of  average  meteoric 
material,  or  meteoric  stone-iron ;  this  should  be  surrounded 
by  a  shell  of  meteoric  nickel-iron,  the  accretion  of  which 
took  place  after  the  lighter  meteoric  stone  had  been  largely 
thrown  into  elliptic  orbits  of  high  eccentricity  or  into  hyper- 
bolic orbits,  and  an  outer  shell  built  up  slowly  from  those 
meteoric  stones  which  later  return  from  their  eccentric 
elliptical  orbits  or  arrive  from  other  nuclei  along  hyperbolic 
orbits.  Under  this  theory  the  several  shells  should  be  ar- 


THE  NATURE  OF  THE  EARTH'S  INTERIOR 


19 


ranged  as  in  fig.  1,  which  seems  to  be  entirely  in  harmony 
with  Gutenberg's  observations.  If  we  assume  the  central 
core  to  have  a  radius  of  3500  km.  and  an  average  density  of 
6.9,  the  intermediate  shell  of  nickel-iron  a  thickness  of  1700 
km.  and  a  density  of  7.6,  and  the  outer  shell  of  meteoric 
stone  a  thickness  of  1200  km.  and  an  average  density  of  3.6, 
the  whole  should  have  the  density  of  the  earth  planet, 
namely  5.6.  A  survey  of  the  different  theories  of  earth  con- 
stitution may  be  obtained  from  fig.  2. 


Gutenberq 
I9K 

FIG.  2. — Diagrams  to  illustrate  the  different  theories  concerning  the  earth's 

interior 

That  a  group  of  terrestrial  rocks  of  igneous  origin  which 
in  density  approach  nearest  to  the  meteoric  stones,  should 
also  be  nearest  them  in  composition  and  in  some  of  its  sub- 
divisions be  considered  to  overlap  the  series  of  stony 
meteorites,  is  certainly  a  fact  of  great  importance.  No  less 
than  five  sub-groups  of  igneous  rocks  are  common  to  terres- 
trial and  celestial  rock  series.  The  presumption  that  the 
terrestrial  examples  of  these  types  have  an  origin  deeper 
down  in  the  earth's  mass  than  their  less  basic  relatives,  and 
that  those  of  celestial  origin  belonged  to  the  near-surface 
portions  of  the  bodies  from  which  they  came,  seems  to  be 
supported  by  all  the  facts  known  to  us.  The  simplest  ex- 


20        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

planation  for  the  differences  which  we  observe  in  relatively 
large  meteorites  is  one  that  regards  them  as  the  detached 
parts  of  disrupted  planetary  bodies,  of  which  the  stones 
represent  the  surface  and  near-surface  fragments,  and  the 
irons  the  deeper  interior  portions.  Inasmuch  as  free  oxygen 
has  not  been  found  in  meteorites,  and  original  water  but 
doubtfully,  the  bodies  from  which  they  came  are  naturally 
regarded  as  much  smaller  than  the  earth  of  the  present  time 
— in  fact  too  small  to  hold  an  atmosphere.  It  is  probably 
for  this  reason  that  no  rocks  at  all  resembling  our  sedimen- 
tary series,  such  as  sandstones,  shales,  and  limestones,  have 
reached  us  from  space. 

Even  though  the  meteoric  irons  reveal  no  structure  to 
indicate  a  former  fused  condition,  meteoric  stones  not  in- 
frequently contain  rock  glass  such  as  is  common  in  our 
terrestrial  igneous  rocks  of  surface  origin,  and  this  is  gen- 
erally taken  to  indicate  a  rapid  congelation  from  a  fused 
condition.  Quite  local  surface  melting  from  in  fall  of 
meteoric  bodies  is  quite  in  harmony  with  our  conception  of 
the  mechanics  of  the  process. 

More  and  more  since  the  promulgation  of  the  planet- 
esimal  theory  of  earth  evolution,  the  tendency  has  been  to 
regard  meteoric  bodies  which  reach  the  earth's  surface  as 
constituting  "world  stuff,"  though  in  its  larger  masses  this 
material  probably  supplies  examples  of  parts  of  disrupted 
bodies  which  have  been  themselves  aggregated  out  of 
planetesimals.  In  a  quite  recent  paper  Chamberlin  has 
stated  emphatically:  "The  masses  of  meteorites  and  their 
methods  of  infall  throw  a  flood  of  light  on  the  sizes  and 
modes  of  infall  of  the  planetesimals,  for  by  this  interpreta- 
tion they  are  bodies  of  like  origin  and  like  general  condi- 
tions." 

Both  in  the  internal  and  in  the  surface  structure  of 
meteorites  there  are  found  indications  of  the  distinctly 
limited  size  of  planetesimal  bodies.  Insofar  as  the  meteor- 
ites are  not  composed  of  nickel-iron,  90  percent  of  them 


THE  NATURE  OF  THE  EARTH'S  INTERIOR      21 

are  possessed  of  a  nodular  or  chondritic  structure,  the 
chondrules  being  found  imbedded  in  fragmentary  material, 
and  the  surfaces  of  these  nodules  like  those  of  the  surround- 
ing fragments  generally  giving  evidence  of  impact  and  attri- 
tion (a — c,  Fig.  3).  The  large  individual  crystals  of  meteoric 
minerals,  such  as  the  olivines  in  the  famous  Pallas  iron,  pre- 
sent a  peculiar  rounded  surface  with  all  crystal  angles  re- 
moved as  though  by  attrition,  and  with  only  the  central  por- 
tions of  the  crystal  faces  still  remaining  (at  left  in  fig.  3). 
The  chondrules  which  are  such  a  dominant  feature  of  the 
meteoric  stones  seldom  exceed  an  inch  in  diameter. 

"It  does  not  appear  safe/'  says  Professor  Chamberlin, 
"to  assume  that  even  the  innermost  core  of  the  earth  took 


FIG.  3. — Structures  of  meteorites.  Left,  olivine  crystal  from  the  Pallas  iron 
on  which  the  oval  areas  are  the  only  remnants  of  the  crystal  faces; 
middle,  chondrule  with  veinlets  of  rock  glass;  right,  chondrules  with 
eccentric  spherulitic  structure 

on  the  liquid  state,  even  at  the  outset,  or,  if  it  did,  that  it 
long  retained  that  state,  however  congenial  to  inherited 
predilections  such  an  inference  may  be."  In  all  but  the 
earlier  stages  the  planetesimal  bodies  would  have  to  plunge 
through  an  envelope  of  satellites  and  an  atmosphere,  and 
today  the  velocity  of  meteorites  approaching  the  earth's 
surface  is  first  counteracted  until  reduced  to  zero  by  air 
resistance,  after  which  the  bodies  fall  by  gravity  and  attain 
such  moderate  velocities  only  that  they  are  generally  but 
little  imbedded  in  the  earth.  Cases  are  upon  record  where 
meteorites  falling  upon  thin  ice  have  not  even  broken  it. 
Chamberlin  has  assumed  that  the  earth  during  all  stages 
of  its  final  shaping  was  an  elastic  solid  of  high  rigidity. 

Since  it  is  not  possible  to  assert  positively  that  the  gen- 
eration of  heat  from  infall  of  meteorites  of  large  size  may 


22        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

not  in  some  stages  at  least  of  the  process  of  planet  growth 
have  been  sufficient  to  fuse  the  entire  mass,  it  becomes  im- 
portant to  consider  the  problem  in  what  manner  congelation 
would  have  taken  place  under  such  circumstances.  Barrell 
has  indeed  modified  the  Chamberlin  hypothesis  to  the  ex- 
tent of  assuming  the  infall  of  bodies  of  planetary  dimen- 
sions, instead  of  those  having  a  magnitude  not  much  greater 
than  sand  or  dust;  and  such  infall,  he  says,  would  be 
"sufficient  to  produce  in  the  growing  earth  a  molten  state 
at  least  in  the  outer  portions."  Against  such  a  conclusion  is 
the  fact  that  the  chondrules  or  largest  fragments  in  meteor- 
ites seldom  exceed  an  inch  in  diameter. 

Kelvin,  as  long  ago  as  1878,  had  considered  this  subject 
and  reached  the  conclusion  that  "the  heat  caused  by  the  col- 
lisions, after  the  earth  had  grown  to  be  anything  approach- 
ing its  present  size,  would  be  far  more  than  sufficient  to 
melt  the  material  falling  in." 

As  was  pointed  out  in  the  last  chapter,  the  general  belief 
in  the  formation  of  a  crust  over  the  supposed  molten  globe 
is  at  bottom  based  upon  analogy  with  the  formation  of  ice 
upon  the  surface  of  water.  The  first  to  examine  the  sub- 
ject scientifically  with  a  view  to  determining  whether  solid 
igneous  rock  or  its  molten  equivalent  has  the  greater  density 
was  Bischoff,  who  in  1843  made  simple  and  indecisive  ex- 
periments which  led  him  to  believe  that  three  types  of 
igneous  rock — basalt,  granite,  and  trachyte — are  on  the 
average  about  ten  percent  denser  than  the  corresponding 
melts.  Kelvin,  in  1876,  in  his  "Review  of  Evidence  Regard- 
ing the  Physical  Condition  of  the  Earth,"  wrote,  "I  may 
say,  with  some  degree  of  confidence,  that  whatever  may  be 
the  relative  densities  of  rock,  solid  and  melted,  at  or  about 
the  temperature  of  liquefaction,  it  is,  I  think,  probable  that 
cold  solid  rock  is  denser  than  hot  melted  rock,"  and  he  adds 
in  another  place,  speaking  of  the  earth,  "As  soon  as  the 
surface  began  to  freeze,  and  to  freeze  in  sufficient  quantity 
not  to  be  floated  up  by  mere  superficial  solidified  foam,  the 


THE  NATURE  OF  THE  EARTH'S  INTERIOR  23 

mass  of  the  rock  would  fall  downwards  towards  the  center. 
More  would  then  solidify  at  the  surface.  This  also  would 
fall  down,  and  the  same  thing  would  go  on  again  and 
again." 

In  an  elaborate  series  of  difficult  experiments,  Barus  in 
1893  first  proved  that  basalt,  the  commonest  type  of  vol- 
canic rock,  contracts  on  passing  into  the  solid  from  the 
liquid  state.  These  studies  had  been  carried  out  upon  the 
initiative  of  Clarence  King,  who  was  making  a  special  study 
of  the  nature  of  the  earth's  interior  and  its  probable  age. 
King's  conclusion  from  his  studies  was  that  at  present  "we 
have  no  warrant  for  extending  the  earth's  age  beyond  24 
millions  of  years"  and  that  upon  this  assumption  an  origi- 
nally molten  earth  would  have  solidified  from  the  center 
outward  to  the  crust  and  become  perfectly  solid. 

The  fact  that  blocks  of  solid  basaltic  lava  float  upon  the 
molten  material,  not  only  in  the  experiments  but,  appar- 
ently, in  the  open  lava  lake  of  Halemaumau  (Kilauea)  in 
the  Hawaiian  Islands,  has  been  responsible  for  some  mis- 
apprehension. At  Kilauea  such  phenomena  are  of  frequent 
occurrence,  but  careful  observation  has  revealed  the  fact 
that  these  apparently  floating  rock  islands  are  in  reality  at- 
tached to  the  walls  or  to  the  bottom  of  the  lava  lake.  The 
disrupted  lava  crust  founders  in  the  lake. 

Wholly  apart  from  the  question  of  what  the  condition 
of  the  earth's  interior  may  have  been  in  the  past,  we  have 
ample  evidence  that  its  condition  today  is  at  least  as  rigid 
as  a  ball  of  steel  of  the  same  size,  since  otherwise  it  would 
suffer  distortions  of  large  amplitude  through  the  action  of 
the  tides  produced  by  the  varying  attractions  of  moon  and 
sun.  Kelvin,  who  was  the  first  to  demonstrate  this,  said: 
"Suppose  the  earth  to  consist  of  a  thin  shell  or  crust  en- 
closing or  floating  on  a  vast  interior  of  molten  matter.  The 
liquid  interior  would  tend  to  yield  freely  to  the  tide-gen- 
erating influence  of  the  sun  and  moon.  The  consequence 
would  be  that  the  exterior  crust  would  be  acted  on  by  forces 


24        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

which,  unless  it  were  of  preternaturally  rigid  material  .  .  . 
it  would  be  unable  to  resist.  The  crust  would  then  be  sub- 
ject to  upheavals  and  depressions  taking  place  in  time  with 
the  revolutions  of  the  sun  and  moon.  If  the  crust  yielded 
perfectly,  there  would  be  no  tides  of  the  sea,  no  rising  and 
falling  relatively  to  the  land  at  all.  The  water  would  go 
up  and  down  with  the  land,  and  there  would  be  no  relative 
movement ;  and  in  proportion  as  the  crust  is  less  or  more 
rigid,  the  tides  would  be  more  or  less  diminished  in  magni- 
tude." 

The  earth  tide  though  inappreciable  even  when  measured 
by  what  are  called  precise  methods,  has  none  the  less  been 
measured  by  Michelson  with  the  aid  of  a  special  device  of 
which  his  interferometer  is  an  important  part.  These 
studies  have  confirmed  Kelvin's  statements  and  shown  that 
the  measured  and  the  calculated  earth  tides  are  almost 
identical,  and,  as  Michelson  says,  are  of  value  "in  refuting 
the  old  notion  that  the  internal  temperature,  sufficiently 
high  to  melt  most  of  the  materials  constituting  the  earth's 
crust,  necessarily  involves  a  fluid  or  semi-fluid  earth  sup- 
porting a  relatively  thin  solid  crust."  The  coefficients  of 
both  viscosity  and  rigidity,  Michelson  found  to  be  of  the 
order  of,  and  perhaps  exceeding,  those  of  solid  steel.  Since 
at  the  temperatures  believed  to  obtain  within  the -earth's 
core  all  substances  would  be  fluid,  "it  follows,"  he  says, 
"that  pressure  increases  the  rigidity  and  the  viscosity,  at 
least  of  the  substances  which  form  the  body  of  the  earth." 

Experiments  by  Bridgman  carried  out  in  the  Jefferson 
Physical  Laboratory  of  Harvard  University  at  pressures  as 
high  as  30,000  atmospheres,  have  confirmed  this  and  clearly 
indicated  that  the  fusing  point  of  bodies  is  elevated  enor- 
mously with  the  increase  of  pressure,  thus  adding  important 
confirmation  of  the  now  accepted  view  that  the  earth's 
interior  is  kept  rigid,  and  presumably  solid,  by  the  pressure 
due  to  the  weight  of  superincumbent  material.  It  follows, 
of  course,  that  if  pressure  were  to  be  locally  removed  in  any 


THE  NATURE  OF  THE  EARTH'S  INTERIOR      25 

place,  that  portion  of  the  mass  would  become  fused  and 
produce  a  local  reservoir  of  molten  rock.* 

There  are  two  other  considerations  which  confirm  the 
view  that  the  earth's  interior  is  today  essentially  rigid  like 
steel.  The  first  of  these  is  based  upon  the  fact  that  the 
earth  continues  to  spin  about  its  axis  of  rotation;  while 
the  other  is  based  upon  the  high  transmission  velocity  of 
vibrations  which  are  carried  through  the  body  of  the  earth 
at  the  time  of  great  earthquakes. 

Kelvin  was  accustomed  to  illustrate  the  incapacity  of  a 
fluid  body  to  spin  by  attempting  to  spin  both  boiled  and 
raw  eggs  upon  the  lecture  table  by  twirling  them  between 
the  palms  of  the  hands.  The  hard-boiled  egg,  having  an 
essentially  rigid  or  solid  interior,  is  easily  made  to  spin, 
though  all  attempts  to  spin  the  raw  egg  with  its  essentially 
liquid  interior  are  unavailing.  Internal  friction  between 
the  different  liquid  shells  stops  the  motion  imparted  to  the 
raw  egg. 

But  the  spinning  earth  like  the  spinning  top  or  the  boiled 
egg,  does  not  maintain  its  rotating  axis  in  a  stationary  posi- 
tion. The  axis  has  a  leisurely  tilting  (precessional)  rota- 
tion as  compared  with  the  rotation  of  the  body  about  its 
axis,  and  the  earth  body  is  similarly  deformed  by  amounts 
which  are  measurable  in  the  precise  methods  of  the  astrono- 
mer. These  are  measured  in  the  variations  of  latitude 
according  to  the  formula  of  Euler.  The  earth  was  in  these 
studies  at  first  assumed  to  be  perfectly  rigid,  which  would 
require  a  Euler  period  of  rotation  of  ten  months.  It  was 
only  after  Newcomb  had  suggested  that  a  period  of  four- 

*The  above  conclusions  apply  of  course  to  uniform  (hydrostatic)  pres- 
sure. Pressures  which  are  non-uniform,  of  the  nature  of  a  shear,  follow 
the  law  that  the  fusing  point  is  always  lowered  with  increasing  pressure, 
as  is  well  brought  out  especially  by  the  studies  of  Johnston  and  Adams 
at  the  Geophysical  Laboratory  in  Washington.  It  does  not  seem  possible, 
however,  that  shearing  stresses  can  ever  exceed  the  elastic  limit  of  the 
materials,  and  must  therefore  be  relatively  insignificant  when  compared 
with  the  average  hydrostatic  pressure  at  great  depths  below  the  surface. 
The  effects  mentioned  by  Johnston  and  Adams  refer  to  a  stress  system 
in  which  there  is  a  discontinuity  of  the  normal  component  of  stress  across 
a  surface  of  separation. 


26        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

teen  months  be  taken,  which  would  be  in  correspondence 
not  with  perfect  rigidity  but  with  that  of  steel,  that  the 
observed  data  were  found  to  compare  well  with  the  com- 
puted one.  The  studies,  now  confirmed  by  Michelson's 
experimental  data,  gave  us  the  first  actual  measurement  of 
earth  rigidity  in  terms  of  solid  bodies  whose  properties  are 
known  to  us. 

When  seismographs,  or  earthquake  recording  devices,  had 
been  so  perfected  as  to  register  earthquakes  from  points  as 
remote  from  the  observing  station  as  the  antipodes,  a 
means  was  discovered  for  measuring  the  translational 
velocity  of  the  vibrations  which  are  carried  directly  through 
the  earth,  as  well  as  those  which  travel  along  its  surface. 
Now  the  velocity  of  such  forms  of  harmonic  motion  is  well 
known  to  be  a  measure  of  both  the  density  and  the  elasticity 
of  the  medium  traversed,  and  the  determined  average 
velocity  in  excess  of  ten  kilometers  per  second  for  compres- 
sional  waves  passing  through  the  earth  shows  clearly  that 
its  interior  is  not  only  rigid  but  has  an  elasticity  at  least  as 
great  as  that  of  the  best  tool  steel.  It  can  hardly  be  other 
than  significant  that  by  several  different  methods  the 
rigidity  and  elasticity  of  the  earth's  core  should  be  found 
to  approximate  so  closely  to  that  of  steel,  and  it  argues 
strongly  for  a  general  identity  of  the  core  material  with 
that  of  the  nickel-iron  meteorites. 

LITERATURE 

GUSTAV  BISCHOFF.  Versuche  die  Kontraktion  zur  bestimmen,  welche 
geschmolzene  Massen  erleiden  wenn  sie  in  dem  festen  Zustande  ueber- 
gehen  und  krystallinische  Gesteine  bilden,  etc.,  Neues  Jahrbuch  f. 
Min.,  etc.,  1843,  pp.  1-54. 

LORD  KELVIN.  Popular  lectures  and  addresses,  vol.  2,  Geology  and  gen- 
eral physics.  Macmillan. 

CARL  BARUS.  High  temperature  work  in  igneous  fusion  and  ebullition, 
chiefly  in  relation  to  pressure,  Bull.  103  U.  S.  Geol.  Surv.,  pp.  1-57  pis. 
1-9,  chap.  2  especially. 

T.  A.  JAGGAR,  JR.  Volcanologic  Investigations  at  Kilauea,  Am.  Jour.  Sci. 
(4),  vol.  44,  1917,  pp.  171-172. 

CLARENCE  KING.  The  age  of  the  earth,  Am.  Jour.  Sci.  (3),  vol.  45,  1893, 
pp.  1-20. 

CHAMBERLIN  AND  SALISBURY.    Geology,  vol.  2,  chap.  2. 


THE  NATURE  OF  THE  EARTH'S  INTERIOR       27 

JOSEPH  BARRELL.  The  strength  of  the  earth's  crust,  reprinted  from  articles 
in  the  Journal  of  Geology,  vols.  22  and  23,  1914-15. 

0.  C.  FARRINGTON.  Meteorites,  their  structure,  composition,  and  terres- 
trial relations,  Chicago,  1913,  p.  225. 

A.  A.  MICHELSON.  Preliminary  results  of  measurements  of  the  rigidity 
of  the  earth,  Jour.  Geol.,  vol.  22,  1914,  pp.  97-134. 

A.  A.  MICHELSON  and  HENRY  G.  GALE.  The  rigidity  of  the  earth,  Jour. 
Geol.,  vol.  27,  1919,  pp.  585-601. 

T.  C.  CHAMBERLIN,  H.  F.  REID,  J.  F.  HAYFORD,  and  FRANK  SCHLESINGER. 
The  earth;  its  figure,  dimensions,  and  the  constitution  of  its  interior, 
Proc.  Am.  Phil.  Soc.,  vol.  54,  1915,  pp.  279-308.  Reprinted  in  Smith- 
sonian Report  for  1916,  pp.  225-254. 

FRANK.  D.  ADAMS  and  ERNEST  G.  COKER.  An  investigation  into  the  elastic 
constants  of  rocks,  more  especially  with  reference  to  cubic  compressi- 
bility, Am.  Jour.  Sci.,  vol.  22,  1906,  pp.  95-123. 

R.  D.  OLDHAM.  The  constitution  of  the  interior  of  the  earth  as  revealed 
by  earthquakes,  Quart.  Jour.  Geol.  Soc.,  vol.  62,  1906,  pp.  456-473. 

JOHN  JOHNSTON  and  L.  H.  ADAMS.  On  the  effect  of  hign  pressures  on 
the  physical  and  chemical  behavior  of  solids,  Am.  Jour.  Sci.,  vol.  35, 
1913,  pp.  205-253. 

P.  W.  BRIDGMAN.  The  failure  of  cavities  in  crystals  and  rocks  under 
pressure,  ibid,  vol.  45,  1918,  pp.  243-268. 

GEORGE  P.  MERRILL.  The  composition  and  structure  of  meteorites  com- 
pared with  that  of  terrestrial  rocks,  Smithsonian  Report  for  1917, 
1919,  pp.  175-188,  pis.  1-9. 

T.  C.  CHAMBERLIN.  Diastrophism  and  the  formative  processes,  XI,  Se- 
lective segregation  of  material  in  the  formation  of  the  earth  and  its 
neighbors,  Jour.  Geol.,  vol.  28,  1920,  pp.  126-157. 

A.  L.  DAY,  R.  B.  SOSMAN  and  J.  C.  HOSTETTER.    The  determination  of 

mineral  and  rock  densities  at  high  temperatures,  Am.  Jour.  Sci.   (4), 

vol.  37,  1914,  pp.  1-39. 
E.  WIECHERT.     Ueber  die  Massenvertheilung  im  Innen  der  Erde,  Nachr. 

v.  d.  k.  Gessel.  d.  Wiss.  z.  Gottingen,  1897,  pp.  221-243. 
E.  WIECHERT.    Our  present  knowledge  of  the  earth,  Smith.  Rept.,  1908, 

1909,  pp.  431-449. 
T.  C.  CHAMBERLIN  and  R.  D.  SALISBURY.    Geology,  vol.  2,  1905,  pp.  133-138. 

B.  GUTENBERG.    Ueber  Erdbebenwellen,  VIIA,  Beobachtungen  an  Regis- 
trierungen  von  Fernbeben  in   Gottingen  und   Folgerungen   uber   die 
Konstitution  des  Erdkorpers,  Nach  v.  d.  k.  Gessel.  d.  Wiss.  z.  Gott- 
ingen, 1914,  pp.  125-176. 

T.  C.  CHAMBERLIN.  The  self-compression  of  the  earth  and  related  prob- 
lems, Yearbook  No.  19,  Carnegie  Institute  of  Washington,  1920,  pp. 
366-383. 


CHAPTER  III 
THE   SOURCE   OF   VOLCANIC   LAVA 

FEW  geologists  could  be  found  today  who  would  dissent 
from  the  view  that,  whatever  may  have  been  its  condition 
in  the  past,  the  interior  of  the  earth,  regarded  as  a  whole, 
is  possessed  of  a  rigidity  roughly  comparable  with  that  of 
solid  steel.  Since,  however,  molten  rock  accompanied  by 
gases  periodically  reaches  the  surface  of  the  earth  through 
volcanic  vents,  we  have  no  recourse  but  to  conclude  that 
the  source  of  this  lava  and  gas  is  to  be  found  in  pockets,  or 
maculce,  and  it  seems  probable  that  these  are  situated  rela- 
tively near  to  the  surface  of  the  earth. 

It  is  a  fundamental  conception  of  geology  that  the  earth 
has  long  been  and  still  is  losing  heat  through  radiation  into 
the  space  surrounding  it.  Though  this  idea  grew  up  and 
was  fostered  by  the  now  abandoned  conception  of  a  liquid 
interior,  it  does  not  lack  support  from  observations  made 
in  deep  mining  shafts  and  in  deep  borings  that  have  pene- 
trated somewhat  further  into  the  lithosphere.  Up  to  the 
depth  to  which  observations  have  now  been  carried,  about 
a  mile  and  a  half,  the  temperature  continues  to  increase; 
and  a  composite  made  from  somewhat  variant  data  gives  us 
as  perhaps  the  best  value  for  the  rate  of  increase  of  tem- 
perature with  depth — geothermic  gradient — 1°  Fahrenheit 
for  each  60  feet  of  descent.  //  this  gradient  be  conceived  to 
continue  without  change  in  the  direction  of  the  earths 
center,  a  temperature  at  which  all  rocks  would  fuse  and 
even  be  vaporized  under  surface  conditions  would  even- 
tually be  reached,  and  a  temperature  sufficient  for  fusion 
would  be  attained  above  the  depth  of  twenty  miles.  Most 
rocks  would  fuse  at  a  depth  of  fifteen  miles  on  the  basis  of 

28 


THE  SOURCE  OF  VOLCANIC  LAVA  29 

dry  conditions  only.  Since  fusion  is  not  general  it  must  be 
explained,  as  has  already  been  pointed  out,  by  the  elevation 
of  the  fusing  point  of  the  materials  with  the  augmenting 
pressure  upon  them.  That  increase  of  temperature  with 
depth  continues  at  least  to  the  level  necessary  for  local 
liquefaction  of  rock,  it  is  necessary  to  assume  in  order  to 
account  for  the  existence  of  lava  at  all.  Under  the  influence 
of  the  water  present,  so-called  aqueous  fusion,  depths  much 
less  than  those  above  given  for  dry  fusion  would  be  required. 

The  rate  of  lowering  of  the  fusion  point  of  rock  with  re- 
spect to  the  increasing  water  content  is  almost  startling, 
and  this  is  augmented  rapidly  with  the  percentage  of  the 
associated  water.  It  has  been  shown  that  amorphous  po- 
tassium silicate  having  the  composition  of  K2Si2O5  when 
containing  between  ten  and  twenty-five  percent  of  admixed 
water  is  at  ordinary  temperatures  a  hard  glass;  whereas 
with  increasing  proportions  of  water  it  becomes,  first,  a  stiff 
paste,  and,  finally,  a  very  viscous  solution  resembling  water 
glass.  When  entirely  free  from  water  this  substance  fuses 
at  a  temperature  of  1015°C.,  whereas  with  only  eight  per- 
cent of  admixed  water  the  melting  point  is  lowered  to  only 
about  one-half  that  figure,  or  500°C. 

The  temperature  of  lava  as  it  exudes  at  the  earth's  surface 
is  generally  from  800°  to  1500°C.,  but  the  higher  tempera- 
tures we  now  know  to  be  in  large  part  due  to  the  elevation 
of  temperature  from  combination  of  its  occluded  gases  as 
they  separate  from  the  mass  and  mingle  near  the  margins 
on  approach  to  the  surface,  and,  further,  to  oxidation  when 
they  are  released  into  the  atmosphere.  This  intensive  union 
of  unstable  gases  as  they  separate  from  magmas  has  been 
aptly  described  as  blowpiping,  from  its  close  resemblance 
to  the  processes  which  go  on  in  the  hot  flame  of  a  blowpipe. 

Studies  of  the  very  siliceous  vein-rock  which  is  known  as 
pegmatite  and  which  is  usually  a  marginal  phase  of  granitic 
magma,  have  shown  that  all  gradations  exist  between  a  true 
rock  melt  and  a  true  solution  product.  With  all  these 


30        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

facts  in  mind  it  is  not  considered  unreasonable  to  suppose 
that  rock  may  be  fused  to  form  magma  at  depths  well  within 
the  earth's  rind  of  sediments,  and  probably  not  more  than 
six  miles  below  the  surface.  The  depth  to  which  water  ex- 
tends downward  into  the  lithosphere  has  been  the  subject 
of  much  controversy,  but  its  amount  must  rapidly  decrease 
below  the  depth  of  five  or  six  miles. 

With  the  necessary  concessions  for  the  formation  of  local 
pockets  of  lava,  there  is  no  good  reason  to  assume  that  at 
greater  depths  the  geothermic  gradient  either  increases  or 
diminishes,  or  that  it  is  not  absolutely  arrested  at  some  very 
moderate  depth.  Of  this  whole  matter  we  lack  knowledge, 
and  the  pseudo-scientific  estimations  of  temperature  at  the 
earth's  center,  obtained  by  multiplying  the  gradient  for 
the  near-surface  zone  by  the  length  of  the  earth  radius,  are 
of  a  kind  which  cannot  be  defended,  but  which  has  been 
far  too  common  in  the  history  of  science.  A  lesson  might 
here  by  learned  from  the  experience  of  the  great  physicist 
Helmholtz  who  averred  that  the  earth's  atmosphere  could 
not  extend  upward  from  the  surface  beyond  an  elevation 
of  27  or  28  kilometers,  upon  the  ground  that  the  aerothermic 
gradient  within  the  lower  9  kilometers  of  the  atmosphere 
—all  that  had  been  at  that  time  explored — required  that  if 
this  gradient  continue  upward  the  absolute  zero  of  tempera- 
ture would  be  attained  at  an  elevation  of  28  kilometers. 
Subsequent  exploration  of  the  air  has  revealed  that  in 
middle  latitudes  the  aerothermic  gradient  is  suddenly  ar- 
rested and  gives  place  to  essentially  isothermal  conditions 
at  the  level  of  a  fairly  definite  ceiling  only  about  two  kilo- 
meters beyond  the  extreme  elevation  to  which  soundings 
had  been  carried  at  the  tune  of  Helmholtz's  statement. 

Whenever,  from  any  cause  whatever,  local  relief  from 
the  pressure  of  overlying  rock  is  secured  within  the  litho- 
sphere, provided  the  rock  temperature  equals  or  exceeds 
that  necessary  for  rock  fusion  under  the  reduced  pressure,  a 
local  chamber  of  molten  rock  (magma)  may  come  into 


THE  SOURCE  OF  VOLCANIC  LAVA          31 

existence.  We  shall  defer  the  consideration  of  the  condi- 
tions under  which  such  relief  of  pressure  may  be  obtained, 
in  order  to  take  up  first  the  kind  of  rock  which  is  likely  to 
undergo  fusion  and  thus  produce  a  magma  pocket. 

The  proper  method  of  attack  upon  this  problem  is  ob- 
viously to  examine  into  the  constitution  of  rocks  of  molten 
(so-called  igneous)  origin,  both  those  which  consolidate 
within  the  lithosphere  (e.g.,  granite),  and  those  which  issue 
as  lava  through  a  vent  at  the  earth's  surface  (e.g.,  basalt). 
It  will  then  be  necessary  to  examine  into  the  composition 
of  the  three  main  types  of  stratified  rocks,  rocks  which  have 
been  formed  by  the  process  of  sedimentation,  and  described 
as  sandstones,  shales,  or  limestones.  These  rocks  are  made 
up  of  the  re-deposited  and  re-cemented  debris  from  the 
fragmentation  of  earlier  rocks,  for  whose  formation  the 
atmosphere  and  hydrosphere  were  indispensable. 

The  parts  of  the  series  of  earth  sediments  are  never  all 
present  together  at  any  one  place,  and  though  when  pieced 
together  they  would  make  up  a  pile  on  some  estimates  over 
sixty  miles  in  thickness,  it  is  probable  that  they  never  ex- 
ceed a  tenth  of  that  figure  in  any  one  place. 

Our  first  problem  is  evidently  to  examine  into  the  con- 
stitution of  the  rocks  which  have  congealed  from  molten 
material  and  are  described  as  igneous.  A  composite  made 
from  practically  all  of  them  that  have  been  carefully  and 
completely  analyzed,  over  five  thousand  in  all,  is  given 
below  in  column  1.  Recalculated  so  as  to  include  the  eight 
principal  constituents  only,  we  derive  the  figures  of  column 
2,  opposite  which  under  column  3  are  printed  the  so-called 
molecular  ratios:  (See  table  on  page  32.) 

Of  greatest  importance  among  the  constituents  are  ob- 
viously silica  and  alumina,  which  together  make  up  nearly 
two-thirds  of  the  entire  rock.  The  range  of  content  of 
these  constituents  in  the  entire  series  of  igneous  rocks  is  of 
the  greatest  interest  and  significance.  If  we  except  perhaps 
a  dozen  rare  rocks  which  occur  in  any  considerable  quantity, 


32        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


Silica    

Average 
igneous  rock 
(Clarke  and 
Washington, 
1920) 

59  09 

Recalculated 
for  Principal 
Constituents 

61  02 

Alumina 

1535 

1585 

Ferric  Oxide   

308 

392 

Ferrous   Oxide    

380 

348 

Magnesia    

349 

360 

Lime 

508 

524 

Sodium  Oxide   

384 

396 

Potassium  Oxide   

313 

323 

Water     

1  14 

Titanium  Dioxide 

105 

Phosphorus  Pentoxide 

030 

Manganous  Oxide    

0.125 

Carbon  Dioxide    

0102 

Zirconium   Dioxide    
Sulphur 

0.039 
0053 

Chlorine    

0.056 

Fluorine    

0078 

Chromium   Sesquioxide 

0056 

Vanadium   Sesquioxide    

0.032 

Nickelous   Oxide 

0025 

Barium  Oxide   

0055 

Strontium  Oxide       

0022 

Lithium  Oxide    . 

0.007 

Molecular 
Ratios 

1.017 
.155 
.025 
.044 
.090 
.094 
.055 
0.34 


100.000 


100.00 


the  silica  in  no  case  much  exceeds  eighty  percent  or  falls 
much  below  thirty  percent.  The  alumina,  likewise,  shows 
a  strictly  limited  range  and  is  practically  always  less  than 
twenty-five  percent. 

Magma  pockets  may  of  course  be  conceived  to  be  located 
either  within  the  outermost  rind  of  the  earth  which  is  com- 
posed of  a  complex  of  igneous  and  sedimentary  rocks,  or  in 
the  shell  immediately  beneath,  which  we  conceive  to  have 
originated  from  the  infall  of  meteoric  stone.  It  has  already 
been  shown  in  the  last  chapter  that  as  regards  chemical 
composition  the  lightest  of  the  meteoric  stones  are  essen- 
tially identical  with  the  densest  of  our  igneous  rocks.  Less 
than  five  percent  of  the  igneous  rocks  fall,  however,  within 
this  category,  and  we  are  forced  to  conclude  that,  generally 
speaking,  igneous  rocks  have  been  largely  derived  from  local 
fusion  of  sediments  included  in  the  outermost  skin  or  rind 


THE  SOURCE  OF  VOLCANIC  LAVA  33 

of  the  lithosphere.  Insofar,  however,  as  they  may  consist 
of  refused  igneous  rocks,  their  composition  will  probably  be 
but  slightly  changed  in  the  process  of  fusion.  For  such 
cases  the  problem  is  merely  carried  one  step  farther  back 
to  still  earlier  rocks  out  of  which  these  particular  igneous 
rocks  were  formed. 

The  study  of  the  composition  of  sedimentary  rocks  is, 
on  the  other  hand,  of  the  utmost  interest  and  significance 
in  this  connection.  By  far  the  greater  number  of  these 
rocks  appear  to  have  been  deposited  on  the  floor  of  the 
ocean  relatively  near  shore  and  where  their  materials  were 
either  brought  down  suspended  in  river  water  or  were 
wrested  from  the  shore  itself  by  storm  waves.  All  the 
material,  whether  of  one  or  the  other  mode  of  origin  has 
been  distributed  by  the  tides  and  the  shore  currents  over 
the  ocean  bottom  and  there  laid  down  along  with  the  limey 
remains  of  marine  organisms.  Far  out  from  shore  beyond 
the  reach  of  most  of  the  land-derived  sediments  the  depos- 
its are  dominantly  calcareous,  derived  from  shells  and  other 
organic  remains,  and  these  when  hardened  into  rock  produce 
limestones.  Nearer  the  shore  the  finer  land-derived  sedi- 
ments are  deposited  as  muds  admixed  with  more  or  less 
organic  matter,  and  these  when  transformed  into  hard  rock 
are  known  as  shales  or  slates,  by  far  the  most  abundant 
of  all  the  sedimentary  rock  types.  Still  nearer  the  shore 
where  the  strong  currents  raised  in  connection  with  the 
breakers  can  transport  the  larger  rock  fragments,  we  find 
the  sands  which  harden  into  sandstones  and  often  the 
shingle  whose  pebbles  make  up  the  pudding-stones  or 
conglomerates. 

As  regards  their  relative  abundance  it  has  been  estimated 
that  the  shales  constitute  from  sixty  to  eighty  percent  of  all 
sedimentary  rocks,  with  the  limestones  and  sandstones 
each  from  ten  to  twenty  percent.  The  continents  being 
surrounded  generally  by  the  so-called  continental  shelves 
over  which  the  water  gradually  deepens  to  about  six  hun- 


34       EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


dred  feet  at  the  outer  margin,  it  follows  that  wherever  thick 
deposits  of  marine  sediments  have  been  laid  down  in  con- 
tinuous process,  the  shelf  which  forms  the  floor  of  that 
deposition  must  have  been  steadily  depressed  while  the  sea 
advanced  over  the  land,  and  that  the  outermost  zone  where 
limestone  sediments  can  alone  be  deposited  must  have 
moved  progressively  landward  over  that  of  mud  deposition, 
and  the  mud-depositing  area  likewise  over  the  sands  of  the 
earlier  shore.  It  is  for  this  reason  that  shales  so  generally 
overlie  sandstones,  and  that  limestones  so  often  cap  a  sedi- 
mentary series. 

Let  us  now  examine  into  the  average  composition  of  each 
of  these  great  series  of  sedimentary  rocks.  It  is  evident  that 
the  principal  constituent  of  sand,  quartz  or  silica,  must 
dominate  in  the  sandstones,  whereas  lime  and  magnesia, 
together  with  carbonic  oxide  will  be  the  chief  constituents 
of  the  limestones.  The  shales  are  of  more  complex  compo- 
sition and  will  occupy  intermediate  positions  with  a  com- 
position bearing  a  close  relation  to  the  main  constituent  of 
mud — clay,  a  substance  high  in  alumina  and  water. 

Average  Composition  of  Sedimentary  Rock  Types 


SiO, 

A1208 

Fe203 

FeO 

MgO 

CaO 

Na.0 


K20 


Igneous  Rocks 

Shale 

Sandstone 

Limestone 

(Composite 
of  5179) 

(Composite 
of  134*) 

(Composite 
of  624) 

(Composite 
of  843) 

61.02 

64.18 

86.27 

16.03 

1.017 

1.070 

1.438 

0.267 

15.85 

16.91 

5.58 

2.13 

0.155 

0.165 

0.054 

0.021 

3.92 

4.44 

1.29 

1.10 

0.025 

0.027 

0.008 

0.007 

3.48 

3.05 

0.59 

0.044 

0.042 

0.008 

3.60 

3.07 

0.88 

10.30 

0.090 

0.077 

0.022 

0.257 

5.24 

3.41 

3.45 

69.13 

0.094 

0.060 

0.061 

1.234 

3.96 

1.39 

0.65 

0.56 

0.055 

0.022 

0.011 

0.009 

3.23 

3.56 

1.29 

0.75 

0.034 

0.038 

0.014 

0.008 

100.00  100.00 

Additional  analyses  here  added  by  the  author. 


100.00 


100.00 


THE  SOURCE  OF  VOLCANIC  LAVA 


35 


At  the  suggestion  of  Dr.  G.  K.  Gilbert,  the  average  com- 
position of  each  of  the  main  types  of  sedimentary  rock  was 
some  years  ago  determined  in  the  laboratory  of  the  U.  S. 
Geological  Survey  at  Washington,  with  results  which  are 


Average  Limestone 

(Composite  of 


FIG.  4. — Composite  rock  diagrams  for  the  main  classes  of  rocks 

given  below  in  the  following  table;  the  less  abundant  con- 
stituents being  here  for  the  moment  left  out  of  considera- 
tion and  the  analyses  recalculated  for  the  eight  principal 
constituents. 

The  close  identity  between  the  analyses  of  the  average 


37 


38       EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

shale  and  the  average  igneous  rock  is  certainly  remarkable. 
On  the  other  hand,  there  is  no  correspondence  whatever 
between  the  average  igneous  rock  and  the  average  sand- 
stone or  the  average  limestone.  Since  the  eye  notes  and 
retains  form  characteristics  far  better  than  it  does  mere 
numerical  figures,  these  relationships  appear  to  better 
advantage  upon  the  simple  diagrams  of  Fig.  4  in  which  the 
key  illustrates  the  method  employed  for  expressing  the 
molecular  proportions  of  the  constituents. 

It  should,  however,  be  pointed  out  that  there  is  one 
respect  in  which  the  average  analysis  of  igneous  rocks  does 
not  correctly  represent  the  average  chemical  composition  of 
the  rocks  themselves  in  the  proportion  in  which  these  are 
found  in  the  earth's  outermost  shell.  Those  rock  types 
which  occur  in  greatest  abundance,  granite  and  its  close 
relatives  chemically,  are  so  well  known  to  us  that  there  is 
little  occasion  to  have  expensive  chemical  analyses  made 
from  them.  It  is  the  exceptional  rock  type  for  which  it  is 
necessary  to  prepare  chemical  analyses,  and  this  is  notably 
true  of  the  alkaline  rocks  with  their  lower  silica  content  and 
their  higher  percentages  of  the  alkalies  and  lime.  This  fact 
may  well  in  large  part  account  for  the  lower  silica  and  the 
higher  lime  and  alkali  percentages  of  the  average  igneous 
rock  as  compared  with  the  average  shale;  but  there  are 
doubtless  other  reasons  also  for  this  difference,  some  of 
which  will  be  pointed  out. 

An  investigation  carried  out  by  the  writer  in  order  to 
determine  whether  the  relationships  revealed  by  the  average 
rock  types  applied  also  to  the  individuals,  has  supplied  a 
somewhat  striking  confirmation  which  is  probably  causal 
in  its  origin.  It  was  noted,  however,  that  the  rarer  igneous 
rock  types,  the  ultra-basic  groups  of  the  peridotites  and 
pyroxenites  which  overlap  the  celestial  and  terrestrial  rock 
series,  are  not  represented  by  any  shales  that  have  been 
analyzed.  The  diagram  of  analyses  from  a  selected  group  of 
volcanic  igneous  rocks  when  compared  with  those  of  a  group 
of  shales  taken  almost  at  random  (Figs.  5  and  6)  bring  out 


THE  SOURCE  OF  VOLCANIC  LAVA  39 

the  lower  silica,  lime  and  soda  content  of  the  igneous  rocks 
just  as  do  the  composites. 

Notwithstanding    the    rather    remarkable    approach    to 
identity  shown  both  in  the  average  and  in  the  individual 
analyses  of  shales  and  igneous  rocks,  there  are  certain  slight 
divergencies  which  may  or  may  not  have  significance.    The 
silica  of  the  average  shale  analysis  is  nearly  3  per  cent  higher 
than  that  of  the  average  igneous  rock,  and  the  oxides  of  lime 
and  soda  are  both  slightly  higher  in  the  composite  igneous 
rocks,  being,  namely,  in  excess  by  1.83  and  1.57  per  cents 
respectively.    As  already  explained,  this  may  be  wholly  due 
to  natural  selection  in  the  analyses  that  were  usually  made. 
It  is,  further,  not  difficult  to  account  for  these  small 
differences  upon  the  theory  advanced,  on  the  ground  of  the 
rather  small  number  of  analyses  which  have  been  combined 
for  an  average  shale — 134;  but  so  far  as  the  oxides  of  lime 
and  soda  are  concerned,  there  is  a  rather  obvious  reason  why 
such  differences  should  appear.    In  the  process  of  ascending 
to  the  surface,  magma  will,  after  the  pressures  have  become 
sufficiently  reduced,  more  or  less  completely,  calcine  the 
limestone  which  it  encounters  with  the  result  of  absorbing 
the  oxide  of  lime  after  separation  of  the  carbon  dioxide. 
Similarly  from  the  considerable  quantities  of  common  salt 
which  are  locked  up  in  certain  of  the  marine  sediments, 
oxide  of  sodium  will  be  assimilated  after  separation  of  the 
chlorine.    The  increments  of  both  lime  and  sodium  oxides 
should  in  consequence  be  larger  in  the  effusive  igneous 
rocks  than  in  the  deeper  seated  types.    Composites  made 
from  plutonic,  dike  and  effusive  igneous  rock  types  which 
were  prepared  by  the  author  some  years  ago  indicate  that 
this  view  is  not  without  some  warrant,  at  least  as  respects 
the  content  of  lime  oxide.    The  figures  obtained  are: 

Comparison  of  Calcium  and  Sodium  Oxides  in  the  Plutonic  and  Effusive 

Igneous  Rocks 

Plutonic                  Dike  Effusive 

(Composite          (Composite  (Composite 

of  1166)                 of  119)  of  1123) 

CaO    .  4.39  5.85  6.63 

Na*0    .  3.55  4.45 


40        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

The  question  of  fusibility  is  clearly  one  which  has  impor- 
tance in  connection  with  this  problem  of  magma  origin,  and 
it  may  well  be  of  determinative  value.  Though  we  are 
without  data  concerning  the  range  of  fusibility  of  sand- 
stones, shales,  and  limestones,  we  are  probably  warranted 
in  drawing  some  conclusions  respecting  their  fusing  points 
from  those  of  the  materials  which  constitute  the  chief  con- 
stituent in  each. 

Quartz,  the  principal  constituent  of  sandstones,  is  when 
pure  one  of  the  most  difficultly  fusible  substances  which  is 
known,  since  it  melts  in  the  neighborhood  of  1700°  C.,  a 
value  higher  by  several  hundred  degrees  than  any  tempera- 
ture which  has  been  obtained  from  volcanic  lavas.  Calcium 
carbonate,  the  basis  of  limestone,  according  to  Boecke  fuses 
at  pressures  above  110  atmospheres  at  1289°  C.  Clay,  the 
basis  of  shale  and  slate,  has  so  large  an  importance  in  the 
industries  that  its  fusibility  properties  are  pretty  well 
known.  The  most  refractory  clays  known,  which  are  of 
course  low  in  fluxing  impurities,  having  fusing  temperatures 
which  may  equal  or  exceed  that  of  calcite,  but  the  common 
types  heated  under  dry  conditions  begin  to  fuse  at  about 
1000°  C.,  and  the  range  of  their  fusibility  is  about  the  same 
as  that  of  actual  lavas  which  reach  the  earth's  surface. 
Lava  temperatures  have  been  measured  in  volcanoes  and 
found  to  range  from  about  970°  C.  to  white  heat  (1500°  C.), 
but  the  higher  values  appear  to  be,  not  the  temperatures  of 
the  lava  mass  as  a  whole  but  those  obtained  where  unstable 
gases  are  being  liberated  into  the  atmosphere  through  ori- 
fices and  producing  intense  blowpiping  effects.  It  is  prob- 
able that  the  mass  temperatures  of  lavas  seldom  go  above 
1200°  C. 

There  is  another  problem  of  much  importance  for  which 
a  ready  solution  is  found  upon  this  assumption  that  magma 
is  produced  by  the  fusion  of  shale.  It  has  often  been 
pointed  out  that  a  close  relationship  in  composition,  a  so- 
called  "consanguinity,"  connects  the  lavas  which  are 


THE  SOURCE  OF  VOLCANIC  LAVA 


41 


erupted  at  localities  widely  distributed  throughout  a  some- 
what broad  area.  Within  a  neighboring  province  similar 
co-relationships  may  be  established  to  connect  eruptions  of 
lava  at  its  different  vents,  though  with  a  common  marked 
difference  setting  them  apart  from  the  lavas  of  the  province 
first  mentioned  or  with  those  of  other  areas.  Such  prov- 
inces, within  each  of  which  one  general  type,  or  related 
types,  of  lava,  or  magma,  is  encountered,  are  commonly 
referred  to  as  petrographic  provinces  of  comagmatic  regions. 
It  has  been  customary  to  ascribe  the  affinities  of  the  lavas 
within  any  province  to  the  existence  of  a  common  subter- 
ranean lava  reservoir,  and  the  differences  to  lateral  mag- 


FIG.  7. — Maps  to  indicate  how  a  characteristic  petrographic  province  may 
develop  from  local  fusions  of  shale 

matic  differentiation  subsequent  to  the  formation  of  the 
reservoir.  Because  of  the  wide  extent  of  such  provinces, 
the  assumed  magmatic  differentiation  in  a  lateral  direction, 
where  obviously  gravity  could  play  but  a  small  role,  has 
made  a  large  demand  upon  the  imagination.  So  soon  as  we 
account  for  the  origin  of  magmas  in  the  local  fusion  of 
shale,  it  becomes  unnecessary  to  assume  a  common  reser- 
voir extending  throughout  the  province,  and  the  affinities 
and  the  differences  as  well  which  are  observed  to  character- 
ize the  lavas  of  the  different  vents,  are  all  alike  explainable 
by  the  common  deposit  of  muddy  sediment  from  which  the 
shale  resulted.  The  differences  have  also  been  introduced 
by  tide  and  shore  current  at  the  time  of  deposition. 


42        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

A  petrographic  province  characterized  by  soda  rhyolites 
and  situated  in  south  central  Wisconsin  shows  the  relation- 
ships and  the  differences  in  composition  which  are  brought 
out  in  the  first  of  the  following  tables.  When  compared 
with  the  set  of  analyses  of  slate  in  the  second  table  repre- 
senting various  localities  within  the  Vermont-New  York 
slate  belt,  it  will  appear  how  natural  such  relationships  of 
composition  would  be  upon  the  theory  advanced.  Upon  the 
same  scale  the  petrographic  province  and  the  slate  belt  are 
represented  in  the  maps  of  Fig.  7. 

IGNEOUS  ROCKS  FROM  PETROGRAPHIC  PROVINCE  OF 
SOUTH  CENTRAL  WISCONSIN 

Obser-  En-       Tay- 

vatory    Mar-      Bara-     deav-     lor's        Mar- 
Hill    quette      boo          or       Farm     cellon    Alloa    Utley 

SiO2    74.46  73.09  71.24  72.80  78.23  79.03  71.14  73.09 

A12O3    15.28  15.40  12.20  15.50  11.11  13.23  19.58  13.43 

Fe2O3    1.95  0.65  1.71  2.04  1.73        0.34  1.25  2.57 

FeO    0.74  2.10  5.44  0.60  1.03        0.18  0.88 

MgO     0.08  0.12  0.13  0.08  Tr.         0.7  0.37  1.03 

CaO     0.92  1.74  0.98  0.52  0.28        0.25  2.14  2.29 

Na«O     2.57  4.57  4.29  5.70  3.44        3.95  2.34  3.85 

K20    3.01  2.01  1.86  2.52  4.08       2.28  2.62  1.58 

SLATES   FROM   DIFFERENT  LOCALITIES   IN   THE  VERMONT- 
NEW  YORK  SLATE  BELT 

Poult-  Castle-   Hamp-    Gran-   Janes-   Race- 

ney  Pawlet   Benson     ton         ton        ville       ville       ville 

SiOa    59.27  67.76  59.70  60.96  67.61  67.55  67.89  63.88 

A1203    18.81  14.12  16.98  16.15  13.20  12.59  11.03  9.77 

Fe2O3    1.12  0.81  0.52  5.16  5.36  5.61  1.47  3.86 

FeO    6.58  4.71  4.88  2.54  1.20  1.24  3.81  1.44 

MgO     2.21  2.38  3.23  3.06  3.20  3.27  4.57  5.37 

CaO    0.42  0.63  1.27  0.71  0.11  0.26  1.43  3.53 

NasO    1.88  1.39  1.35  1.50  0.67  0.61  0.77  0.20 

K2O    3.75  3.52  3.77  5.01  4.45  4.13  2.82  3.45 

LITERATURE 

F.  W.  CLARKE.  Analyses  of  rocks  from  the  laboratory  of  the  U.  S.  Geo- 
logical Survey,  1880-1903,  Bull.  U.  S.  Geol.  Surv.,  no.  228,  1904,  pp. 
20-21. 

H.  S.  WASHINGTON.  The  distribution  of  the  elements  in  igneous  rocks, 
Trans.  Am.  Inst.  Min.  Eng.,  1908,  pp.  1-30. 

WILLIAM  H.  HOBBS.  Some  considerations  concerning  the  place  and  the 
origin  of  lava  maculae,  Gerland's  Beitrage  zur  Geophysik.  vol.  12, 
1913,  pp.  330-361. 


THE  SOURCE  OF  VOLCANIC  LAVA  43 

WILLIAM   H.   HOBBS.    Variations  in   composition   of  pelitic   sediments  in 

relation   to   magmatic   differentiation,   Comptes   Rendus  XII   Session 

Congres  Geologique   International    (Stockholm),   1912,   pp.  241-246. 
JOSEPH   P.  IDDINGS.    The  problem   of  volcanism,  Yale  University  Press, 

1914,  pp.  101-105. 
H.  S.  WASHINGTON.    The  chemical  analyses  of  igneous  rocks,  Prof.  Paper. 

U.  S.  Geol.  Surv.,  no.  14,  1903,  pp.  495  (revised  and  expanded  as  Prof. 

Paper  no.  99,  1917,  pp.  1201). 
H.   S.   WASHINGTON.    The   chemistry   of  the   earth's   crust,  Jour.   Frank. 

Inst.,  vol.  190,  1920,  pp.  757-815. 
N.  H.  DARTON.     Geothermal  data  of  the  United  States,  including  many 

original  determinations  of  underground  temperature,  Bull.  701,  U.  S. 

Geol.  Surv.,  1920,  p.  97. 


CHAPTER  IV 

THE   ORIGIN   OF  POCKETS   OF   MOLTEN  ROCK 

IT  has  been  shown  that  the  rigidity  of  the  earth  must  be 
generally  maintained  today  under  conditions  of  internal 
temperature  which  locally  at  least  and  relatively  near  to  the 
surface  are  sufficiently  high  for  at  least  the  aqueous  fusion 
of  rock.  This  rigidity  is  further  assumed  to  exist  in  con- 
sequence of  compression  due  to  load,  which  compression 
within  the  very  considerable  limits  to  which  tests  have  been 
carried  raises  enormously  the  melting  point  of  such  solids 
as  have  been  tested. 

It  follows  that  though  the  rock  temperature  be  sufficient 
to  cause  fusion  under  surface  conditions  of  pressure,  within 
the  lithosphere  rock  will  melt  at  those  places  only  at 
which  the  load  of  superincumbent  material  has  in  some 
way  been  either  wholly  or  partially  lifted  off— the  load  must 
be  at  least  partially  supported  without  bearing  upon  the 
portion  of  the  mass  which  is  to  pass  into  the  fused  condi- 
tion. Our  problem  is,  therefore,  to  examine:  (a)  the  forces 
present  and  competent  to  produce  such  structures  as  will 
lift  the  load  above  a  shale  formation  and  (b)  the  nature 
of  the  structures  which  they  develop. 

It  has  been  customary  among  geologists  to  regard  the 
lithosphere  as  continually  losing  heat  through  radiation  into 
the  space  about  it  (so-called  secular  cooling)  and  in  conse- 
quence diminishing  in  volume.  The  outermost  earth  shell 
having  long  since  adjusted  itself  to  the  temperatures  imme- 
diately above  it,  it  is  the  interior  portion  of  the  planet  which 
is  now  shrinking,  and  the  adjustments  of  the  shell  neces- 
sary to  close  in  about  its  shrinking  core  under  the  impelling 
force  of  gravitation,  must  induce  compressional  stresses 
within  the  shell. 

44 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK  45 

The  view  thus  expressed  has  in  recent  years  been  assailed 
by  certain  physicists  who  have  set  up  the  claim  that  suffi- 
cient heat  is  being  evolved  from  emanations  of  radium  to 
either  partially  or  wholly  offset  the  earth's  losses  of  heat 
through  secular  cooling.  The  geologist  can  afford  to  ignore 
this  challenge  of  the  physicist,  since  whatever  may  be  the 
computed  relative  losses  and  gains  in  heat  energy,  there  is 
testimony  supplied  from  the  study  of  earthquakes  which 
is  fortunately  unequivocal  and  leaves  the  geologist  no 
recourse  but  to  conclude  that  the  volume  of  the  earth  is 
today  diminishing. 

Before  taking  up  this  evidence,  it  seems  advisable  to  first 
state  the  case  as  revealed  in  vertical  sections  transverse  to 
the  strata.  Deformations  which  take  place  in  the  process 
of  mechanical  adjustment  within  that  portion  of  the  litho- 
sphere  that  is  open  to  our  inspection  are  of  two  kinds: 
namely,  flexuring  or  folding,  and  block  displacement  or 
faulting.  Quite  obviously  the  first  mentioned  type  of 
adjustment  must,  in  and  by  itself,  supply  a  proof  of  shrink- 
ing, for  the  layers  which  are  folded  cannot  possibly  cover  as 
large  an  area  as  they  did  before  folding  had  occurred.  This 
is  made  clear  by  Fig.  8,  A  and  B.  It  has  been  estimated  that 
in  the  folding  of  the  Alps  the  area  of  sediments  involved  has 
been  reduced  to  about  one-sixth  of  its  former  width. 

It  now  must  be  considered  in  how  far  shrinking  of  area 
through  flexuring  of  the  strata  may  be  offset  by  adjustments 
of  block  faulting  within  regions  where  folding  has  not  taken 
place.  Such  adjustments  may  obviously  be  of  any  one  of 
three  types,  since  vertical  planes,  or  those  perpendicular  to 
a  spherical  surface  like  that  of  the  earth  converge  down- 
ward, and  they  may  be  so  adjusted  that  the  center  of  vol- 
ume of  the  disturbed  region  has  been  moved;  a,  farther 
away  from  the  earth's  center,  b,  brought  nearer  to  it,  or,  c, 
kept  at  the -same  distance  as  before.  Exaggerating  grossly 
the  amount  of  curvature  of  the  earth's  surface  for  the  sake 
of  demonstration,  these  three  cases  are  illustrated  by  C,  D, 


46        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

and  E  of  Fig.  8.  It  is  obvious  that  they  must  correspond 
respectively  to  a  net  shrinking,  to  a  net  swelling,  and  to 
an  unaltered  volume  of  the  earth  during  the  adjustment, 
so  far  at  least  as  the  affected  region  alone  is  concerned.  It 
should  be  pointed  out,  however,  that  compared  to  the 
changes  in  area  which  result  from  complex  folding,  the 
measure  of  these  changes  in  area  is  relatively  small. 
Now  it  is  well  known  that  the  so-called  "normal"  type 


(A) 


FIG.  8. — Diagrams  to  illustrate  the  effects  of  folding  and  of  faulting  of 
different  types  upon  the  superficial  area  of  the  earth 

of  block  faulting  on  other  than  vertical  planes  is  that  repre- 
sented in  Fig.  9,  in  which  the  blocks  must  tend  to  express 
an  extension  of  the  surface,  and  without  careful  quantita- 
tive comparisons  of  the  sum  total  of  contraction  within 
folded  regions  with  the  total  of  expansion  in  block  faulted 
regions — a  matter  for  the  distant  future,  if  it  may  indeed  be 
considered  a  soluble  problem — we  are  forced  to  admit  that 
from  this  angle  the  solution  of  the  problem  is  now  inde- 
terminate. 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK  47 

It  is  the  investigation  of  earthquakes  which  has  supplied 
us  with  an  answer  to  the  question  which  fortunately  admits 
of  no  equivocation.  It  is  today  recognized  that  earthquakes 
are  the  manifestations  of  the  frequent  sudden  adjustments 
of  the  earth's  surface  portions  to  accumulating  stresses 
within,  and  with  each  relief  so  obtained  is  brought  about  a 
larger  or  smaller  change  of  form  in  the  surface.  The  dis- 
tribution of  earthquakes  shows  that  for  the  land  areas 
earthquakes  occur  chiefly  where  mountains  are  growing- 
rearing  their  heads — and  where  the  surface  is  therefore 
generally  considered  to  be  expanding  rather  than  contract- 
ing. Now  the  really  surprising  observation  has  been  made 
that  after  the  earthquake  the  surface  has  really  contracted, 


FIG.  9. — Section  across  a  normal  fault  showing  how  it  may  increase  the 
surface  area  of  the  earth 

other  and  neighboring  portions  of  the  surface  having  moved 
in  and  so  permitted  the  original  area  to  rise. 

For  the  present  we  shall  consider  alone  the  evidence  of 
compression  of  the  rocks,  leaving  that  of  the  intruding 
increment  of  surface  to  be  dealt  with  in  a  later  chapter.  A 
study  carried  out  by  the  writer  in  1908  of  the  nature  of  the 
damage  done  to  all  continuous  lines  of  metal,  such  as  rails, 
pipes,  etc.,  and  the  nature  of  the  damage  sustained  by 
bridges  within  regions  affected  by  heavy  earthquakes,  has 
yielded  the  interesting  result  that  these  are  always  such  as 
to  indicate  a  compression  of  the  surface.  Rails  and  pipes 
are  found  to  be  uniformly  buckled  or,  if  parted,  the  dis- 
placed ends  of  the  fractured  metal  are  in  such  positions  as 
to  indicate  a  shortening  of  the  distance.  Bridges  indicate 
a  reduction,  and  often  a  very  large  one,  of  the  distance 


48        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

between  abutments,  but  with  no  corresponding  extension 
indicated  for  the  approaches  upon  either  side  (see  plates 
II-V). 

The  universality  of  this  observation  without  regard  to 
direction  within  the  area  affected,  and  the  general  harmony 
of  the  results  when  tested  out  for  a  number  of  heavy  earth- 
quakes affecting  widely  separated  regions,  force  us  to  the  ' 
conclusion  that  compression  is  the  dominant  type  of  stress 
within  the  earth's  outer  shell,  and  that  this  stress  is  cumu- 
lative, as  is  required  by  the  doctrine  of  continued  secular 
cooling  of  the  interior  of  the  earth  planet. 

An  important  consequence  of  this  constant  compressional 
stress  within  the  earth's  shell  is  that  whenever  the  relief  is 
obtained  by  a  sudden  series  of  displacements  with  the  inevi- 
table result  of  letting  down  the  potential  energy  of  the  sys- 
tem, the  stress  is  again  almost  instantly  renewed  and  the 
vise-like  compression  is  again  set  up  before  the  blocks, 
thrown  beyond  their  natural  positions  of  equilibrium  are 
able  to  return  to  that  position.  Such  return  can  only  be 
accomplished  through  numerous  later  adjustments  in  alter- 
nating directions,  which  have  hence  come  to  be  known  as 
earthquake  after-shocks. 

We  may  now  consider  the  adjustments  which  take  place 
under  each  of  the  separate  heads  of  block  faulting  and  fold- 
ing, as  a  consequence  of  which  may  arise  areas  of  local 
relief  from  pressure  such  as  are  necessary  to  develop  pockets 
of  magma  within  the  lithosphere. 

In  the  case  of  block  faulting  the  denser  massive  strata 
tend  to  be  thrown  by  the  jolting  movement  beyond  their 
position  of  equilibrium,  just  as  does  a  pendulum  when 
released  from  its  extreme  position.  Within  the  upthrown 
blocks  any  heavy  strata  in  a  relatively  high  position,  such 
as  a  lava  capping  or  heavy  limestone  or  sandstone  mem- 
bers, will  tend  to  separate  from  weak  and  relatively  light 
underlying  shale  members  and  as  a  consequence  be  carried 
beyond  them.  The  vise  of  compressive  stresses  almost 


PLATE  II 


Effects  of  earth  movements  upon  rails  and  bridges.  1.  Rails  buckled  in 
approach  to  bridge,  earthquake  of  1891,  Japan.  2.  Nagara  Cowa  railroad 
bridge  dropped  into  river  but  with  all  spans  still  connected,  earthquake  of 
1891,  Japan. 


PLATE  III 


Effects  of  earth  movements  upon  rails.    1.  Buckled  rails,  earthquake  of 
1888,  Charleston,  S.  C.    2.  Buckled  rails,  earthquake  of  1897,  India. 


PLATE  IV 


Effects  of  earth  movements  upon  rails  and  bridges.  1.  Buckled  rails, 
earthquake  of  1906,  California.  2.  Bridge  with  girder  underridden  by  abut- 
ment, earthquake  of  1891,  Japan. 


PLATE  V 


Effects  of  earth  movements  upon  bridges  and  curbing.  1.  Distorted 
abutment,  Salinos  river  bridge,  earthquake  of  1906,  California.  2.  Buckled 
curbing,  earthquake  of  1906,  California. 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK 


49 


instantly  closing  in,  however,  the  massive  upper  strata  may 
in  consequence  be  largely  supported  from  the  sides  and  so 
rest  but  a  portion  of  their  weight  upon  the  shale  below. 
Under  these  conditions  fusion  of  the  shale  will  take  place, 
provided  of  course  the  rock  temperature  and  the  water  con- 


FIG.  10. — Diagrams  to  show  how  block  faulting  may  yield  a  lava  chamber 

tent  make   this  possible  under   the  diminished   pressure 
(Fig.  10). 

Either  slow  subsequent  settlement  of  the  upper  member 
or  a  jolting  return  at  the  time  of  the  earthquake  after-shock, 
will  account  for  the  pressure  from  above  resting  upon  the 
lava  reservoir  and  tending  to  force  its  contents  out  to  the 
surface  along  one  of  the  marginal  faults,  where  it  would  of 


50         EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

course  overflow  the  surface  of  a  neighboring  downthrown 
block  (Fig.  10).  In  Iceland,  in  the  high  plateaus  of  Utah, 
in  the  Great  Basin  of  the  Western  United  States,  and  in  the 
rift  valleys  of  East  Central  Africa,  there  are  numerous  ex- 
amples of  vast  outflows  of  lava  which  have  taken  place 
over  downthrown  blocks  from  marginal  fissures. 

In  the  process  of  rock  folding  there  are  produced  a  suc- 
cession of  alternating  anticlines,  or  arch  folds,  with  syn- 
clines,  or  trough  folds.  The  former  when  composed  of  strong 
rocks,  possess  the  preeminent  competency  of  arched  engi- 
neering structures  to  support  heavy  loads,  provided  the 


FIG.  11. — Model  showing  the  crushing  of  a  shale-like  member  beneath  an 
anticline  of  a  competent  limestone-like  formation 

span  of  the  arch  is  not  too  broad  with  reference  to  its  height. 
To  the  uninitiated  it  will  seem  improbable  that  bodies  nor- 
mally so  rigid  as  rock  should  be  able  to  form  folds  at  all, 
but  that  under  loads  in  excess  of  its  elastic  limit  a  normally 
rigid  body  is  susceptible  of  folding  is  well  established  and 
has  been  aptly  illustrated  in  the  experiments  of  Willis. 

Willis  produced  compositions  from  plaster  of  paris,  bees- 
wax and  Venice  turpentine  mixed  in  varying  proportions, 
and  these  were  shaped  into  layers  of  different  degrees  of 
rigidity  so  as  to  simulate  rock  strata.  Laid  along  the  floor 
of  an  iron  tank  like  a  series  of  strata,  these  layers  could  be 
compressed  from  the  end  by  the  use  of  a  piston  operated 
by  a  screw.  Though  the  layers  yielded  by  fracture  when 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK  51 

compressed  under  the  load  of  air  pressure  alone,  they  under- 
went folding  after  the  manner  of  rocks  within  the  litho- 
sphere  when  subjected  to  a  load  of  shot  equal  to  but  five 
pounds  per  square  inch. 

It  was  found  in  the  experiments  that  the  stronger  layers 
— those  with  larger  proportions  of  plaster  of  paris  in  their 
composition — were  the  ones  which  alone  carried  the  com- 
pressive  stresses  after  the  manner  of  girders,  and  when  the 
strong  members  had  been  placed  above  the  weak  ones — 
simulating  limestone  above  shale — the  relatively  strong 
members  lifted  a  portion  of  the  load  to  form  anticlines, 


Diagram  to  show  position  of  Magma  Chamber 
under  competent  anticline 

FIG.  12. — Diagram  to  show  position  of  a  magma  chamber  formed  under 
a  competent  formation  in  an  anticline 

whereupon  the  weaker  shale  members  below  the  arches,  in- 
stead of  folding,  were  fractured  just  as  though  the  load  had 
not  been  present  at  all  (Fig.  11).  At  a  depth  where  the 
water  content  of  the  rocks  and  the  rock  temperature  to- 
gether supply  a  combination  under  which  shale  is  sus- 
ceptible of  fusing  at  the  reduced  pressure,  the  conditions  are 
met  for  the  development  of  an  anticline  pocket,  or  fold 
pocket,  of  magma  (Fig.  12). 

It  is  characteristic  of  anticlines  that  only  in  the  early 
stages  of  their  growth  have  they  a  symmetrical  form. 
With  increasing  compression  the  side  of  the  arch  which  is 
toward  the  active  compressive  force  progressively  steepens 
and  eventually  bends  under  the  other  and  forms  an  under- 
turned  anticline  which  tends  to  close  up  under  the  con- 


52        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

tinned  squeeze.  Simultaneously  the  magma  pocket  under 
the  continuing  compression  tends  to  be  reduced  in  volume, 
and  will  be  reduced  provided  any  outlet  can  be  found  for 
the  magma.  (See  progressive  sections  of  Fig.  13.) 

Discussion  of  the  manner  of  producing  the  conduit 
through  which  magma  rises  to  the  earth's  surface  will  be 
deferred  until  after  the  gases  of  lavas  have  been  considered. 
The  process  of  compressing  the  lava  pocket  is  simulated  in 
the  operation  of  the  common  form  of  bulb  syringe  from 
which  the  contents  are  expelled  by  compression  within  the 
palm  of  the  hand.  Thus  we  see  that,  as  in  the  case  of  the 
already  described  magma  chambers  which  may  arise  be- 


FIG.  13. — Successive  diagrams  showing  the  manner  of  contraction  of  a 
magma  chamber  through  continuation  of  the  folding  process  with 
expulsion  of  the  lava  through  a  conduit 

neath  the  massive  members  of  upthrown  fault  blocks,  the 
process  of  formation  and  compression  of  the  pocket  supplies 
also  a  simple  solution  for  that  much  mooted  question  of  the 
cause  of  the  rise  of  lava  to  the  surface  of  the  earth. 

It  has  been  said  in  the  foregoing  that  it  is  only  in  the 
earliest  stage  of  a  developing  anticline,  or  arch  fold,  that 
its  form  is  symmetrical,  and  this  symmetrical  and  broadly 
open  arch  of  the  embryonic  stage  thus  represents  the  in- 
itial response  of  the  competent  stratum  in  the  direction  of 
flexure  by  a  local  elevation  of  the  load  resting  upon  it.  It 
is  one  of  the  aims  of  this  treatise  to  show  on  the  basis  of 
observations  that  in  many,  if  not  in  most  instances,  the  in- 
itial form  of  the  anticline  is  not  even  a  symmetrical  arched 
ridge  of  strata,  but  rather  a  linear  series  of  separated  broad 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK  53 

domes  which  later  become  merged  by  growth  into  an  arched 
ridge. 

The  above  described  domes  in  the  strata  enveloping  a 
chamber  of  lava  were  first  described  from  the  Henry  Moun- 
tains in  Utah  under  the  name  of  laccolites  in  a  now  classic 
monograph  by  Mr.  G.  K.  Gilbert.  This  distinguished  Ameri- 
can geologist  believed  the  laccolites  to  be  the  result  of  in- 
trusions of  magma  coming  up  from  below  through  an  un- 
discovered conduit  and  at  a  particular  level  making  their 
way  laterally  between  the  strata  so  as  to  force  the  overlying 
layer  into  a  dome  without  altering  the  position  of  the  un- 
derlying stratum.  The  force  responsible  for  the  structure 
was  thus  made  to  be  the  expansive  hydrostatic  pressure  of 
the  intruding  magma.  Such  structures  have  since  been  de- 
scribed in  considerable  numbers  from  districts  within  which 
the  laccolites  are  generally  found  flanking  the  higher  moun- 
tain ranges.  Such  examples  are  particularly  numerous  in 
the  Rocky  Mountain  region,  where  they  have  been  de- 
scribed in  detail  by  some  of  the  most  competent  of  Ameri- 
can geologists;  and  especially  by  Dr.  Whitman  Cross,  who 
has  published  an  extended  monograph  upon  them. 

All  geologists,  so  far  as  the  writer  is  aware,  have  held  to 
the  original  view  of  Gilbert  that  the  magma  chamber  of 
the  laccolite  represents  an  intrusion  through  a  conduit  un- 
derneath, and  that  the  force  which  elevated  the  roof  was  the 
hydrostatic  pressure  exerted  by  the  magma  itself. 

From  a  somewhat  comprehensive  study  of  the  literature 
of  laccolites,  the  writer  has  been  forced  to  adopt  an  entirely 
different  viewpoint  and  to  see  in  laccolites  the  initial  efforts 
toward  flexuring  within  the  more  competent  strata  when 
these  are  situated  above  a  stratum  of  weak  shale.  Under 
these  conditions  the  competent  stratum  rises  in  domes  and 
the  shale  migrates  inward  to  be  fused  into  a  lava  pocket  as 
already  pointed  out  (see  Figs.  11  and  12). 

It  is  certainly  a  fact  which  calls  for  explanation  on  the 


54        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

part  of  those  who  advocate  an  intrusive  origin  for  laccolites, 
that  with  the  remarkable  simplicity  of  the  structures  and 
the  excellent  exposures  in  section,  the  supposed  conduit 
feeders  have  never  in  a  single  case  been  discovered.  Certain 
other  considerations  of  the  first  importance  which  follow 
naturally  from  the  opposed  conceptions  of  laccolite  origin, 
will  be  easily  understood  from  inspection  of  Fig.  14. 

According  to  Gilbert  and  his  followers  the  stress  condi- 
tion within  the  roof  strata,  at  least  during  the  formation  of 
the  laccolite,  should  be  that  of  tension,  due  to  the  expansive 


UNDISTURBED      LAYERS 


CONTRASTED  VIEWS  OF  ORIGIN  Of  LACCOL1TE5 
FIG.  14 

pressure  of  the  magma  which  is  invoked  to  explain  the 
structure;  and  we  should  therefore  expect  to  see  the  roof 
member  intersected  by  dykes  and  apophyses  of  magma. 
Under  the  view  here  advanced  the  condition  of  stress  within 
the  roof  of  the  laccolite,  instead  of  being  tensional  should 
be  compressional,  since  the  roof  is  believed  to  represent  the 
competent  structure  which  carries  the  earth  compression 
and  is  responsible  for  the  formation  of  the  laccolite.  Fur- 
thermore, an  intrusion  which  is  conceived  to  be  so  vigorous 
and  impelling  as  to  force  up  the  upper  series  of  strata  in  so 
remarkable  a  fashion,  could  hardly  have  failed  to  leave 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK 


55 


some  trace  of  its  energy  in  a  disturbed  condition  of  the  floor 
areas.  Yet  the  evidence  is  in  the  direction  of  a  generally  un- 
disturbed floor,  and  that  keen  observer  who  made  so  many 
of  the  first  studies  of  laccolites,  Mr.  W.  H.  Holmes,  was 


La  Plata 

FIG.  15. — Section  of  the  laccolite  of  the  La  Plata  Mts.  (after  Holmes) 

evidently  greatly  puzzled  by  these  indications  from  his 
observations.  He  not  only  refers  to  it  in  his  descriptions, 
but  in  his  beautifully  drawn  sections  he  habitually  repre- 
sented the  hypothetical  feeding  conduit  as  extremely  nar- 


FLOOR 


LAYER 


ZONE 

LACCOLITH  (GILBERT)' 

leTNCLOSURCS    FLOOR    ROCK) 


LAYER 


LACCOLITE  (HOBB5) 

(ENCLOSURES    3HAL.E) 


FIG.  16. — Contrasted  views  concerning  the  origin  of  laccolites 

row,  possibly  in  part  to  account  for  the  fact  that  he  had  in 
no  case  discovered  it  (Fig.  15). 

Other  no  less  striking  differences  which  characterize  the 
two  conceptions  of  origin,  are  brought  out  in  the  sections 
of  Fig.  16.  Upon  the  Gilbert  conception  of  intrusion,  the 


56        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

enclosures  within  the  laccolite  should  be  either  of  floor  rock 
or  of  materials  from  still  greater  depths  along  the  path  of 
the  conduit.  Upon  the  author's  theory,  rock  enclosures 
within  the  magma  should  generally  be  of  shale,  and  must  in 
most  cases  have  migrated  centripetally  from  the  sides.  It 
may  perhaps  be  possible  to  prove  by  comparative  measure- 
ments that  the  shale  formation  within  which  the  laccolite 
is  formed  increases  in  thickness  as  one  goes  out  from  the 


Loccolitea  of  Judith  Mt3.(Weed  and  Pirsson) 

FIG.  17 

laccolite  margins.  A  study  of  the  literature  of  known 
laccolites  has  supplied  the  remarkable  verification  for  our 
view  of  laccolite  origin  in  the  fact  that  they  appear  only 
within  shale  members  and  quite  generally  beneath  a  compe- 
tent member  of  limestone  or  massive  sandstone  (Figs.  17 
and  18).  The  evidence  for  this  must  be  left  for  presenta- 
tion in  another  place. 

A  very  striking  instance  of  an  apparently  early  stage  of 
laccolite  formation,  within  which  the  shale  has  completed 
its  migration  into  the  dome  but  has  been  as  yet  but  slightly 
fused  is  supplied  by  Holmes'  section  of  a  laccolite  at  Sierra 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK 


57 


La  Sal  in  extreme  northeastern  Utah  (Fig.  19).  A  stage 
in  the  process  not  greatly  different  from  this  is  appar- 
ently illustrated  by  the  Sierra  Abaja,  some  fifty  miles 
east  of  the  Henry  Mountains  in  Utah.  Of  this  occurrence 
Holmes  says:  "The  shales  are  still  found  in  all  parts  of  the 


Massive    Arenaceous    Shales 
Laccollfes       gray  spotted      Limestones 


Limestone  Sandstones 
Map  of  Black  Hills  Laccolites  (Jaggar) 

FIG.  18. — Map  of  the  Black  Hills  laccolites  formed  beneath  a  dome  in 
competent  limestone   (after  Jaggar) 

group  caught  up  in  a  manner  identical  with  that  observed 
in  el  Late  and  Carriso  mountains,  and  the  low  saddles  be- 
tween the  summits  are  invariably  of  these  shales." 

Cross,  in  commenting  upon  Holmes'  description,  says, 
"This  involves  as  a  physical  necessity  the  assimilation  of 


FIG.  19. — Section  of  the  laccolite  of  the  La  Sal  Mts.  with  included  shale 

(after  Holmes) 

shale  by  the  magma,  and  such  is  plainly  Holmes'  meaning, 
though  not  explicitly  stated  in  this  connection." 

The  Sierra  La  Sal  of  northeastern  Utah  the  writer's  pre- 
liminary study  on  the  ground  has  shown  to  be  a  composite 
laccolitic  group  with  the  shale  mantling  over  the  igneous 


58        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

core  and  capping  it  in  the  high  peaks.  The  indications  of 
tension  in  the  roof  of  sediment  called  for  by  the  Gilbert  the- 
ory are  conspicuously  lacking. 

Since  laccolites  pass  by  all  intermediate  gradations  into 
magma  layers  which  are  found  wedged  between  layers  of 
sediments,  it  is  obvious  that  the  same  theory  of  origin  must 
to  a  larger  or  smaller  extent  find  application  to  them,  but 
the  study  has  not  yet  been  extended  in  that  direction. 

The  immense  quantities  of  igneous  rock  which  are  found 
in  the  cores  of  recently  elevated  folded  mountain  ranges 
have  apparently  impressed — amazed,  rather — every  worker 
who  has  studied  them.  The  general  observation  goes  back 
into  the  early  history  of  geology  and  was,  in  fact,  one  of  the 
strong  supports  of  the  famous  "elevation  crater"  theory  of 


Generalized  Section  across  /4ndes( Stein mann) 
FIG.  20 

von  Buch  and  von  Humboldt  in  a  day  when  intrusive  and 
effusive  igneous  masses  were  lumped  rather  indiscriminately 
together.  It  will  be  remembered  that  according  to  this 
theory  the  mountains  were  pushed  up  on  the  back  of  the 
magma  much  as  a  blister  is  raised  upon  the  body,  and,  like 
a  blister,  the  mountain  was  supposed  to  be  a  mere  skin 
filled  out  with  liquid  under  pressure.  Everyone  will  recall 
the  Alps  and  the  Carpathians  with  their  cores  of  granite, 
but  far  more  striking  examples  are  supplied  by  the  Rocky 
Mountains,  the  Sierra  Nevadas  and  the  Coast  Ranges  of 
California,  and  the  great  Cordilleran  system  in  South 
America. 

Steinmann's  generalized  section  across  the  Andes  with 
each  of  the  compressed  anticlines  filled  out  by  a  core  of  gran- 
odioritic  or  andesitic  magma  is  reproduced  in  Fig.  20.  The 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK  59 

batholites  of  granodiorite  and  the  laccolites  of  andesite  are 
alike  non-effusive  rock,  and  they  come  into  contact  only 
with  the  Tithonian  limestones  and  the  Neocomian  clays. 
Steinmann's  description  is  illuminating: 

"The  age  of  the  intrusive  rocks  is  limited  backward  by 
the  post-Cretaceous  folding,  forward  by  the  young  Tertiary, 
and  it  is  proper  to  designate  it  in  general  as  early  or  middle 
Tertiary,  hence,  younger  than  the  principal  period  of  fold- 
ing .  .  .  the  space  for  the  intrusive  masses  was  essentially 
created  by  the  folding  itself.  Neither  the  andesitic  laccolites 
nor  the  batholitic  granodiorites  have  made  a  place  for  them- 
selves by  melting  it  out  in  the  folded  range,  but  they  ob- 
viously fill  previously  formed  openings,  which  does  not  ex- 
clude the  possibility  that  it  has  afterward  been  somewhat 
extended." 

In  the  Sierra  Nevadas  and  the  Coast  Ranges  of  Califor- 
nia, the  core  of  the  ranges  is  occupied  by  an  astounding 
volume  of  granitic  batholites  within  the  folds  to  which 
Lawson  has  forcibly  drawn  attention.  Speaking  of  the  pre- 
Cretaceous  Mesozoic  revolution  which  produced  the  ranges, 
he  says,  "An  essential  feature  of  the  revolution  was  the 
development  of  batholitic  magmas  which  invaded  the  crust, 
replaced  large  portions  of  it,  and  eventually  congealed  as 
plutonic  rock  of  a  prevailingly  acid  character."  The  de- 
velopment of  the  batholites,  he  adds,  "followed  or  accom- 
panied" the  uplift. 

Describing  the  granitic  cores  of  the  eastern  ranges  of  the 
Rocky  Mountains,  these  have  been  rated  by  Van  Hise  as 
pre-Cambrian.  Summarizing  Hayden's  report,  he  says, 
"Each  of  the  great  ranges  of  the  park  are  anticlinal  axes 
with  massive  granite  cores  and  gneissic  granites  inclining 
from  each  side  in  the  form  of  ridges." 

Strangely  enough  the  overlying  rock  strata  generally  lack 
the  basal  conglomerate  which  we  have  every  reason  to  sus- 
pect in  them.  The  weakness  of  the  argument  that  the 
granite  cores  of  such  ranges  antedate  in  age  the  sediments 


60        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


BIS     MORN    ARC 


L  A  R  A  MIE     ARC 


UINTA     ARC 

LEG  EN  D 


Competent  Limestone 
Granite  Cores  of  Anticlines 
Laccolites     ®  Domes 
Arxticline  Arcs 
•©©Volcanoes 


FIG.  21. — Map  of  the  arcs  at  the  front  of  the  Rocky  Mts.  with  their  cores 
of  intrusive  igneous  material 

which  overlie  them,  instead  of  being  contemporaneous  with 
the  folding  of  the  range,  has  never  been  more  forcibly  put 
than  by  Sir  Archibald  Geikie  in  discussing  the  Wasatch 
Range  with  its  granite  core.  Writing  as  long  ago  as  1880, 
he  said : 


THE  ORIGIN  OF  POCKETS  OF  MOLTEN  ROCK  61 

"One  would  naturally  expect  that  if  a  mass  of  strata 
30,000  feet  thick  had  been  laid  down  against  a  steep  slope  of 
land,  its  component  beds  ought  to  be  full  of  the  fragments 
of  that  land.  Each  marginal  belt,  representing  an  old  shore 
line,  should  be  more  or  less  conglomeratic;  at  least  there 
ought  to  be  occasional  zones  of  conglomerate,  just  as  at 
the  present  day  we  have  local  gravel  beaches  on  our  shore. 
.  .  .  But  not  only  have  no  pebbles  of  the  Cottonwood 
granite  been  recorded  as  occurring  in  the  overlying  Paleozoic 
rocks,  it  is  admitted  that  these  rocks  become  meta- 
morphosed as  they  approach  the  granite.  The  natural  in- 
ference to  be  drawn  from  these  facts,  one  might  suppose, 
would  be  that  the  granite  was  later  in  date  than  the  rocks 
overlying  it." 

In  the  Colorado-Wyoming  region  there  is  found  a  series 
of  rather  symmetrical  open  anticlines,  each  with  its  core  of 
massive  granite  and  all  alike  under  a  competent  arch  of  the 
massive  Jura-Trias  limestone,  which  arch  was  elevated  in 
late  Cretaceous  time  (Fig.  21).  When  the  subject  of  the 
sequence  of  these  ranges  has  been  considered  in  a  later 
chapter,  it  will  appear  that  there  are  obvious  gradations 
from  the  laccolites  of  the  Black  Hills  and  Little  Belt  moun- 
tains to  the  elongated  sweeping  curves  of  the  granite  cores 
in  the  Bighorn  and  Laramie  ranges. 

A  question  of  considerable  interest  in  connection  with  this 
theory  of  the  origin  of  laccolites  and  their  related  bodies  of 
igneous  rock,  is  that  of  contact  effects  which  ought  to  be 
looked  for  in  the  enclosing  sediments.  The  theory  of  origin 
assigns  a  common  temperature  to  both  the  shale  which  is 
fused  and  the  sediments  which  remain  unfused  about  its 
margins,  and  except  insofar  as  the  separation  of  gases  from 
the  forming  magma  accomplishes  blowpiping  effects,  there 
seems  to  be  no  good  reason  why  contact  effects  in  the  coun- 
try rock  such  as  belong  to  true  intrusions  should  be  looked 
for  as  a  necessary  condition. 


62        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


LITERATURE 

BAILEY  WILLIS.    The  mechanics  of  Appalachian  structure,  13th  Ann.  Rept. 

U.  S.  Geol.  Surv.,  1893,  pt.  II   pp.  217-281,  pis.  46-96. 
M.  P.  RUDZKI.    Physik  der  Erde,'  Leipzig,  1911,  pp.  118-125. 
WILLIAM  HERBERT  HOBBS.    A  study  of  the  damage  to  bridges  during  earth- 
quakes, Jour.  Geol.,  vol.  16,  1908,  pp.  636-653. 
G.  K.  GILBERT.    Report  on  the  geology  of  the  Henry  Mts.,  U.  S.  Geogr. 

and  Geol.  Surv.  Rocky  Mt.  Region,  1877,  p.  160. 
W.  H.  HOLMES.    Report  on  the  San  Juan  district,  9th  Ann.     Rept.  U.  S. 

Geol.  and  Geogr.  Surv.  Ter.,  1877,  pp.  237-276. 
WHITMAN  CROSS.    The  laccolitic  mountain  groups  of  Colorado,  Utah  and 

Arizona,   14th  Ann.  Rept.  U.  S.  Geol.   Surv.,   1895,  pp.   157-241,  pis. 

7-16. 
G.    STEINMANN.    Gebirgsbildung   und   Massengesteine   in   der   Kordillere 

Siidamerikas,  Geol.  Rundsch.,  vol.  1,  1910,  pp.  13-35. 
A.  C.  LAWSON.    The  Cordilleran  Mesozoic  Revolution,  Jour.  Geol.,  vol. 

1,  1893,  pp.  579-586. 
C.  R.  VAN  HISE.    Correlation  papers,  Archean  and  Algonkian,  Bull.  86, 

U.  S.  Geol.  Surv.,  1892,  chap.  6,  The  Cordilleras,  pi.  9. 
ARCHIBALD   GEIKIE.    On  the   Archean  rocks   of  the   Wasatch   Mts.,  Am. 

Jour.  Sci.  (3),  vol.  19,  1880,  pp.  363-367. 
WALDEMAR  LINDGREN.     Chap.  6  of  Problems  of  American  Geology,  Yale 

Univ.  Press,  1915. 
WILSON  B.  EMERY.    The  Igneous  Geology  of  Carriso  Mountain,  Am.  Jour. 

Sci.  (4),  vol.  42,  1916,  pp.  349-363. 

MALCOLM  RUTHERFORD  THORPE.    Structural  Features  of  the  Abajo  Moun- 
tains, Utah,  ibid.,  vol.  48,  1919,  pp.  379-389. 


CHAPTER  V 

THE  DEPTH  OF  THE  FOLD  POCKETS 

IT  is  known  to  every  one  that  rocks  do  not  under  ordinary 
conditions,  those  with  which  we  are  familiar  at  the  surface 
of  the  earth,  yield  to  compressive  stresses  in  excess  of  their 
strength  by  bending  or  folding,  but  rather  by  fracturing, 
that  is,  crushing.  As  long  ago  as  1878  the  distinguished 
Swiss  geologist  Heim  advanced  the  idea  that  the  process  of 
rock  folding  is  restricted  to  a  zone  within  the  lithosphere 
below  a  depth  of  5,000  metres,  at  which  the  load  of  the 
material  above  is  assumed  to  close  the  rock  pores  and  so 
prevent  opening  of  fractures.  It  is  obviously  of  prime 
importance  for  us  to  consider  what  depth,  if  any,  is  required 
for  folding  to  occur  and  permit  the  formation  of  a  fold 
pocket  of  magma  beneath  a  competent  stratum. 

The  modern  theory  of  a  "zone  of  flow"  separated  from  a 
higher  "zone  of  fracture"  by  an  intermediate  "zone  of  frac- 
ture and  flow"  was  in  1895  elaborated  by  Van  Hise,  who 
declared  that  even  the  strongest  rocks,  the  ones  which  carry 
the  stresses  in  the  folding  process,  would  fold,  or  "flow," 
at  a  depth  of  10,000  metres  or  about  6  miles,  though  shale 
might  be  folded  at  a  depth  of  only  500  metres.  Van  Rise's 
statement  was  tersely  made  in  the  following  words,  "Rocks 
under  less  weight  than  their  ultimate  strength  are  in  the 
zone  of  fracture,  that  is,  when  rocks  under  such  conditions 
are  deformed,  they  break  and  crevices  small  or  great  sepa- 
rate the  broken  parts.  .  .  .  Rocks  buried  to  such  depth  that 
the  weight  of  the  superincumbent  strata  exceeds  their  ulti- 
mate strength  are  in  the  zone  of  plasticity  and  flowage." 

Van  Hise  failed  to  realize  that,  unlike  the  test  pieces 
which  are  subjected  to  crushing  stresses  in  our  engineering 

63 


64        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

testing  laboratories,  rocks  within  the  lithosphere  are  pre- 
vented from  yielding  by  lateral  expansion  when  compressed 
in  a  single  direction,  and  as  a  consequence  the  data  which 
he  derived  possess  small  value.  With  the  use  of  powerful 
modern  hydrostatic  presses,  Professor  Frank  D.  Adams 
carried  out  experiments  in  1912  to  determine  what  pres- 
sures were  necessary  to  close  cavities  which  had  artificially 
been  made  in  test  cylinders  of  rock,  cylinders  which  in  the 
experiments  had  been  first  enclosed  in  nickel-steel  collars 
and  compressed  from  the  ends.  His  results  showed  that  the 
crushing  strength  of  granite  is  under  such  conditions  about 
seven  times  the  usual  figure  given  for  it,  and  Adams 
reached  the  conclusion  that  "under  the  conditions  of  pres- 
sure and  temperature  which  are  believed  to  obtain  within 
the  earth's  crust,  empty  cavities  may  exist  in  granite  to  a 
depth  of  at  least  11  miles.  These  may  extend  to  still 
greater  depths,  and  if  filled  with  water,  gas,  or  vapor  will 
certainly  do  so." 

King  after  a  study  of  the  data  secured  by  Adams  con- 
cluded, "It  is  also  shown  that  as  far  as  hydrostatic  pressure 
in  the  earth's  crust  is  concerned,  a  small  cavity  at  ordinary 
temperatures  will  remain  open  provided  the  depth  does  not 
exceed  a  value  between  17.2  and  20.9  miles." 

It  has  however  been  pointed  out  by  Bridgman  that  in 
these  earlier  experiments  of  Adams  the  friction  of  the  sides 
of  the  rock  cylinders  against  the  walls  of  the  enclosing 
nickel-steel  collars  prevents  the  cavities  from  closing  until 
a  compressive  stress  has  been  registered  which  is  too  large. 
Making  use  of  a  very  ingenious  device  of  his  own  invention 
which  permits  of  true  hydrostatic  conditions  of  compression, 
Bridgman  obtained  values  for  the  crushing  strength  of  rock 
less  than  half  those  which  had  been  obtained  by  Adams.  In 
addition  his  studies  demonstrated  that  stresses  necessary 
to  produce  rupture  under  these  conditions  have  no  connec- 
tion with  any  stresses  and  strains  which  may  be  introduced 
by  crystalline  structure. 


THE  DEPTH  OF  THE  FOLD  POCKETS         65 

Bridgman  reached  the  conclusion  that  pores  will  exist  in 
rocks  under  pressures  such  as  should  be  expected  to  obtain 
at  a  depth  of  twelve  miles  below  the  surface,  so  that  his 
results  are  in  a  general  way  confirmatory  of  those  of  Adams. 
He  says:  "The  results  of  these  collapsing  tests  make  it 
extremely  probable,  however,  that  minute  crevices,  at  least 
large  enough  for  the  percolation  of  liquids,  exist  in  the 
stronger  rocks,  at  depths  corresponding  to  6,000  or  7,000 
kg/cm2,  and  possibly  more."  His  studies  show  also  that 
the  method  of  collapse  of  the  cavities  is  by  a  splintering  of 
their  walls  and  a  filling  of  the  cavity  with  the  fragments. 

In  later  studies  in  this  field  Adams  has  shown  that  cylin- 
ders of  limestone  compressed  from  the  ends  and  enclosed 
within  a  copper  casing  are  able  to  withstand  a  pressure  of 
nearly  150  tons  to  the  square  inch,  whereas  when  tested 
in  the  ordinary  way  they  fail  at  less  than  one-tenth  or  one- 
twentieth  of  that  value— 11,000  to  25,000  pounds  to  the 
square  inch.  Moreover,  it  was  found  that  the  tendency  to 
lose  rigidity  and  acquire  plasticity  is  a  function  of  the  tem- 
perature, and  if  the  experiments  with  granite  may  be 
regarded  as  a  guide,  Adams'  general  figure  for  limestone 
should  be  divided  by  three  if  the  temperature  is  taken  as 
550°  C.  or  that  assumed  to  obtain  at  a  depth  of  fifteen 
miles.  At  ordinary  temperatures  limestone  and  granite  give 
about  the  same  values,  but  limestone  is  unsuited  to  experi- 
ments at  higher  temperatures  because  of  its  tendency  to 
lose  carbon  dioxide  even  at  moderate  temperatures. 

There  are  almost  insuperable  objections  in  the  way  of 
assuming  that  rocks  are  susceptible  of  folding  only  when  at 
depths  of  12  to  15  miles  or  more  within  the  earth's  shell. 
Every  angular  unconformity  with  its  lower  series  of  folded 
rocks  separated  by  an  erosional  surface  from  overlying 
unfolded  layers,  must  upon  this  hypothesis  carry  the  impli- 
cation that,  subsequent  to  the  deposition  of  the  lower  series, 
it  was  by  a  transgression  of  the  sea  buried  by  no  less  than 
12  to  15  miles  of  sediments,  and  then  by  a  regression  of  the 


66        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

sea  of  equal  measure  raised  12-15  miles  before  being  eroded. 
Such  an  assumption  is  of  course  too  preposterous  to  be 
seriously  entertained,  though  it  clearly  follows  if  we  admit 
that  rock  folding  can  alone  take  place  at  depths  below  12 
to  15  miles. 

It  must  be  clear  that  the  factor  which  has  not  received 
due  consideration  is  that  of  time,  which  cannot  be  brought 
into  our  experiments.  Elasticity  and  plasticity  are  in  a 
sense  reciprocal  terms,  since  the  former  property  measures 
the  tendency  to  yield  by  fracture  under  excessive  loads, 
whereas  the  latter  measures  the  tendency  to  yield  by  change 
of  figure,  as  in  folding,  under  the  action  of  the  same  stresses. 
The  inherent  elastic  force  of  a  substance  by  virtue  of  which 
it  resists  any  deforming  force,  diminishes  with  the  time  such 
forces  are  in  operation,  and  for  each  substance  there  is  a 
specific  factor  to  measure  this  called  the  relaxation  time. 
For  steel  and  for  most  strong  rocks,  which  are  very  elastic, 
this  relaxation  time  is  perhaps  measured  in  centuries.  The 
relaxation  time  is  apparently  smaller  the  greater  the  force 
and  the  higher  the  temperature.  Glass  at  300°  C.  has  a 
relaxation  time  of  the  order  of  a  day. 

Any  observing  and  thoughtful  person  who  has  strolled 
through  an  old  graveyard  must  have  been  impressed  by 
the  illustrations  there  furnished  of  the  relaxation  time  of 
marble.  Rectangular  slabs  of  marble  may  be  seen  sup- 
ported in  horizontal  position  by  stone  posts  at  the  four 
corners,  and  it  must  be  clear  that  when  erected  these  slabs, 
two  inches  or  thereabouts  in  thickness,  would  have  sup- 
ported at  their  centers  without  sensible  deflection  a  weight 
several  times  their  own.  Yet,  after  the  lapse  of  centuries 
they  are  found  bent  under  their  own  weight  alone  into  a 
distinct  trough  fold  (Fig.  22). 

There  are,  however,  not  lacking  examples  of  folds  which 
have  been  produced  at  or  near  the  surface  of  the  earth 
through  tangential  compression  induced  by  a  piston-like 
movement  of  an  advancing  glacier  front.  In  such  cases  the 


THE  DEPTH  OF  THE  FOLD  POCKETS 


67 


involvement  of  glacial  materials  within  the  folds  produced, 
together  with  other  evidence,  identifies  the  agent  clearly 
(Fig.  23,  a  and  b).  Also  in  connection  with  the  process 
of  faulting,  a  dragging  or  bending  backward  of  the  strata 


FIG.  22. — Manner  of  formation  of  a  syncline  in  a  marble  slab  under  its 

own  weight  alone 

along  the  fault  plane,  is  not  an  uncommon  observation.  At 
Put-In-Bay  on  South  Bass  Island  in  Lake  Erie  are  found 
caves  which  are  due  to  a  local  elevation  of  limestone  strata 
to  form  domes,  and  subsequent  release  of  the  pressure  from 


FIG.  23.— Anticlines  produced  by  the  pressure  (shove)  of  a  continental 
glacier;  a,  in  Queenstown  shale  on  Lake  Ontario  (Kindle);  b,  in 
coal  bed  in  Illinois  (Sauer) 

below  has  permitted  the  lower  layers  to  settle  back  and 
leave  caves  of  mushroom  form  as  a  result.  In  the  largest 
of  these,  Perry's  Cave,  one  of  the  roof  strata  is  for  a  portion 
of  the  distance  still  in  its  earlier  domed  position,  though  the 


68        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

remaining  portion  has  settled  down  and  again  lies  horizon- 
tally along  the  floor  of  the  cavern.  The  late  Professor 
James  Geikie  described  a  one  inch  marble  slab  enclosed  in  a 
sandstone  casing  in  one  of  the  Scottish  graveyards,  which 
slab  has  become  so  bent  that  an  arm  can  be  inserted  behind 
it.  A  stone  door  jamb  in  the  Alhambra  of  Granada  has 
taken  on  a  strong  curvature  from  load.  All  these  examples, 
and  others  that  might  be  mentioned,  clearly  indicate  that 
bending  of  rock  strata  can  take  place  within  very  moderate 
time  intervals  even  when  the  rocks  are  not  sustaining  a 
heavy  load. 

Who  will  venture  to  predict  what  amount  of  bending 
would  occur  under  the  loads  which  rock  sustains  at  the 
depth  of  a  mile  or  more  within  the  earth's  shell  during  the 
same  period,  or  by  what  factor  this  should  be  multiplied 
to  apply  to  periods  measured  in  thousands,  or  even  millions, 
of  years.  Compared  with  these  periods  of  continued  opera- 
tion of  stresses,  the  times  employed  by  Adams  in  his  experi- 
ments are  so  small  as  to  be  almost  negligible. 

It  would  lead  us  too  far  to  assemble  here  all  the  reasons 
why  any  such  figure  as  a  depth  of  12  to  15  miles  for  the 
upper  surface  of  the  zone  of  folding  is  untenable.  It  will 
be  sufficient  if  we  can  show  that  rock  folding  is  going  on 
today  at  much  higher  levels  and  even  at  the  earth's  surface 
itself. 

A  strong  light  is  thrown  upon  this  problem  by  the  tiltings 
of  reefs  in  the  islands  of  the  coral  seas,  such  as  those  of 
Malaysia  and  Australasia  particularly.  These  recent  for- 
mations, originally  laid  down  in  horizontal  position,  have 
since  been  elevated  and  simultaneously  bent  into  the  forms 
of  folds.  In  the  sections  of  Fig.  24  the  elevated  reefs  are 
marked  by  the  letter  k,  with  in  some  instances  the  relative 
age  of  the  reefs  brought  out  by  a  numerical  figure  which  is 
the  higher  the  later  the  formation  of  the  reef. 

The  eminent  group  of  Dutch  geologists  who  have  devoted 
their  attention  to  a  study  of  the  structure  within  the  island 


THE  DEPTH  OF  THE  FOLD  POCKETS 


69 


l> 


LJ 


111 

*  -a 


II 

!! 


70        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

arcs  of  Malaysia  and  the  Moluccas  have  made  most  impor- 
tant contributions  to  this  subject  by  bringing  out  the  suc- 
cessive steps  in  the  process  of  anticline  formation  within 
the  surface  rock  formations  of  the  islands.  Professor  H.  A. 
Brouwer  has  supplied  the  evidence  for  large  lateral  as  well 
as  vertical  movements  of  the  strata  and  shown  that  succes- 


FIG.  25. — Successive  positions  of  anticline  forming  in  a  rising  island  arc 

(afte  Brouwer) 

sive  positions  of  the  strata  during  anticline  development 
are  represented  by  PABQ  and  P'A'B'Q'  in  Fig.  25,  in  which 
NZ  is  the  level  of  the  sea.  The  curves  represented  are  just 
those  which  correspond  to  successive  stages  of  growing  anti- 
clines on  the  basis  both  of  field  observations  in  mountain 
districts  and  of  the  results  of  experiment.  To  indicate  in 
what  manner  the  coral  growths  betray  such  movements,  we 


RAISED 

REEf  TERRACES 


aectionsto  illustrate  the  nature  and  position 
of  the  reefs  which  result  from  mountain  growth 

FIG.  26. — Manner  of  formation  of  elevated  reef  at  front  and  of  barrier  reef 
at  back  of  a  developing  anticline  arc 

have  added  the  diagrams  of  Fig.  26.  An  upward  motion  of 
the  shore  will  obviously  result  in  successive  elevated  reef 
terraces,  whereas  a  movement  in  the  opposite  direction  will 
result  under  favorable  conditions  in  the  formation  of  barrier 
reefs  or  atolls. 

Though  anticlines  develop  even  at  the  earth's  surface,  and 
doubtless  in  periods  much  shorter  than  those  which  geol- 
ogists are  accustomed  to  admit,  it  none  the  less  seems  prob- 
able that  the  process  of  folding  is  greatly  facilitated  at 


THE  DEPTH  OF  THE  FOLD  POCKETS  71 

greater  depths  where  load  must  be  a  factor  of  importance. 
Expressed  technically,  the  relaxation  time  is  reduced  with 
depth,  and  the  rock  rendered  so  much  the  more  plastic.  In 
some  such  sense  the  basal  idea  of  the  zone  of  flow  may  per- 
haps be  retained.  In  any  case  it  .can  only  be  at  a  depth  suffi- 
cient for  rock  fusion  that  pockets  of  magma  could  form  be- 
neath competent  arches  in  the  manner  already  set  forth 
in  preceding  chapters. 

LITERATURE 

ALBRECHT  HEIM.    Der  Mechanismus  der  Gebirgsbildung,  1878,  p.  31. 

C.  R.  VAN  HISE.    Principles  of  North  American  pre-Cambrian  geology, 

16th  Ann.  Rept.,  U.  S.  Geol.  Surv.,  pt.  I,  1895,  pp.  571-603. 
C.  R.  VAN  HISE.    A  treatise  on  metamorphism,  Mon.  47,  U.  S.  Geol.  Surv., 

1904,  pp.  186-191. 
F.  D.  ADAMS.    An  experimental  contribution  to  the  question  of  the  depth 

of  the  zone  of  flow  in  the  earth's  crust.  Jour.  Geol.,  vol.  20.  1912.  pp. 

97-118. 
L.  V.  KING.    On  the  limiting  strength  of  rocks  under  conditions  of  stress 

existing  in  the  earth's  interior,  ibid.,  pp.  119-138. 
F.  D.  ADAMS  and  E.  G.  COKER.    An  experimental  investigation  into  the 

flow  of  rocks,  First  Paper,  The  Flow  of  Marble,  Am.  Jour.  Sci.,  vol. 

29,  1910,  pp.  465-487. 
JOHN  JOHNSTON  and  L.  H.  ADAMS.    On  the  effect  of  high  pressures  on 

the  physical  and  chemical  behavior  of  solids,  Am.  Jour.  Sci.,  vol.  35, 

1913,  p.  251. 
FRANK  D.  ADAMS  and  J.  AUSTIN  BANCROFT.    On  the  amount  of  internal 

friction  developed  in  rocks  during  deformation  and  on  the  relative 

plasticity  of  different  types  of  rocks,  Jour.  Geol.,  vol.  25,  1917,  pp. 

597-637. 
L.  V.  KING.    On  the  mathematical  theory  of  internal  friction  and  limiting 

strength  of  rocks  under  conditions  of  stress  existing  in  the  interior 

of  the  earth,  ibid.,  pp.  638-658. 
P.   W.  BRIDGMAN.    The  failure   of  cavities  in   crystals  and   rocks  under 

pressure,  Am.  Jour.  Sci.,  vol.  45,  1918,  pp.  243-268. 
P.  W.  BRIDGMAN.    Stress-strain  relations  in  crystalline  cylinders,  ibid.,  pp. 

269-280. 

JAMES  GEIKIE.    Proc.  Roy.  Soc.,  Edinb.,  vol.  10,  1880,  pp.  525-529. 
H.   A.  BRAUER.    On  reef  caps,  K.  Akad.   Wetensch.  Amsterdam,  vol.  21, 

nos.  6-7,  pp.  1-10  (reprint). 

H.  A.  BRAUER.    Fractures  and  faults  near  the  surface  of  moving  geanti- 
clines, I  ibid.,  vol.  23,  No.  4,  pp.  1-7  (reprint). 


CHAPTER  VI 

THE  VAPOES  AND  GASES  OF  LAVA 

In  practically  constant  association  with  the  magma  that 
reaches  the  earth's  surface  are  gases  and  vapors  generally 
in  large  volume,  and  their  liberation  into  the  atmosphere 
with  explosive  combination  and  oxidation  supply  at  least 
most  of  the  energy  which  serves  to  make  volcanic  eruptions 
among  the  most  awe-inspiring  of  natural  phenomena. 

Not  all  the  vapors  and  gases  are  released  into  the  air, 
residual  portions  being  held  in  the  vesicles  of  the  lava; 
other  portions  react  upon  the  rock  or  they  collect  upon  it 
as  solid  sublimates.  These  latter  can  usually  be  driven  off 
by  strongly  heating  the  rock,  by  which  they  often  undergo 
reactions. 

The  gases  and  vapors  which  together  issue  from  an  active 
volcanic  crater,  have  long  been  believed  to  be  in  largest 
part  water,  either  in  the  form  of  superheated  gas  or  of 
vapor;  but  with  the  water  is  usually  associated  one  or  more 
of  the  gases  hydrogen,  carbon  monoxide,  methane,  chlorine, 
and  sulphur  dioxide.  Recent  studies  would  indicate  that 
relative  to  the  water  vapor  the  admixed  gases  play  a  larger 
role  than  has  generally  been  supposed  to  be  the  case. 

In  the  process  of  release  into  the  atmosphere,  those  gases 
which  can  unite  either  with  their  associates  or  with  atmos- 
pheric oxygen,  do  so  and  thus  furnish  much  of  the  energy 
of  crater  explosions.  Hydrogen  burns  to  form  water  vapor, 
the  carbon  monoxide  oxidizes  to  carbon  dioxide,  and  any 
sulphur  to  sulphur  dioxide  and  occasionally  sulphur  tri- 
oxide.  A  portion  of  the  hydrogen  may  unite  with  free 
chlorine  giving  hydrochloric  acid  gas. 

The  "steam"  and  "smoke"  which  are  so  often  described 

72 


THE  VAPORS  AND  GASES  OF  LAVA  73 

in  connection  with  every  active  volcano,  even  during  its 
periods  of  relative  repose  which  separate  grander  eruptions, 
is  now  recognized  to  be  a  mixture  of  steam  with  fine  frag- 
ments of  lava.  This  "smoke"  may  be  either  white,  gray, 
or  black,  the  depth  of  the  shade  indicating  the  proportion 
of  admixed  volcanic  "ash." 

Quite  recently  Brun  has  set  up  the  thesis  that  no  water 
vapor,  or  at  most  very  little,  is  present  in  volcanic  emana- 
tions; the  white  color  of  the  cloud  above  craters  being  in 
his  belief  due,  not  to  aqueous  vapor,  but  to  ammonium 
chloride  or  other  fumes.  This  challenge  of  Brun  to  vol- 
canologists,  supported  as  it  has  been  by  extended  examina- 
tions of  active  volcanoes  and  by  his  own  laboratory  experi- 
ments, has  had  the  effect  of  bringing  out  new  investigations 
by  both  geologists  and  geophysicists,  notably  those  of 
Gautier  and  of  Day  and  Shepherd.  As  a  result  of  these 
studies  one  may  now  state  with, all  confidence  that  the  state- 
ments of  Brun  must  be  modified  though  containing  a  large 
measure  of  truth. 

From  the  surface  of  the  lava  lake  of  Halemaumau  in  the 
crater  of  Kilauea,  Day  and  Shepherd  succeeded  for  the 
first  time  in  drawing  off  the  escaping  vapors  and  gases 
through  an  iron  tube  and  collecting  them  under  such  con- 
ditions as  they  believed  precluded  the  possible  admixture 
of  atmospheric  air.  In  the  first  fifteen  minutes  of  the  ex- 
periment 300  cubic  centimeters  of  water  were  condensed 
in  the  tube.  Later  analysis  showed  the  water  to  vary 
markedly  in  quantity,  namely,  from  18%  to  90%  by  volume 
of  the  entire  mixture.  Later  studies  by  Shepherd  indi- 
cated, however,  that  these  data  were  unreliable  because  of 
the  admixture  with  atmospheric  air.  The  conclusion  which 
is  reached  concerning  Kilauea  at  least,  is  that  little  water  is 
present  in  the  fumes  over  the  crater  with  the  exception  of 
what  has  entered  from  the  atmosphere  within  the  crater 
itself.  Thus  those  early  but  keen  observers,  Green  and  Brig- 
ham,  have  been  confirmed  in  their  opinions.  Like  many 


74        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

others  the  writer  has  observed  the  fume  cloud  drift  away  in 
contact  with  colder  air,  but  without  evidence  of  condensa- 
tion such  as  would  be  necessary  if  it  contained  considerable 
proportions  of  steam. 

In  the  emanations  from  the  quiet  crater  of  Vesuvius  three 
months  and  fifteen  months  respectively  after  the  eruption 
of  1906,  Gautier  found  that  the  proportion  of  water  vapor 
issuing  from  crevices  (fumaroles)  ranged  from  62%  to 
78%  of  all  the  emanations.  These  fumarole  emanations 
issued  at  temperatures  varying  from  250°  to  300  °C.  At 
the  higher  temperatures  at  which  gases  and  vapors  are 
found  in  association  with  lava,  they  must  be  largely  in  a 
dissociated  condition,  and  such  condition  harmonizes  with 
what  we  now  know  of  the  behavior  of  these  gases.  Chief 
among  the  dissociations  which  must  take  place,  is  a  large 
one  of  water  vapor  into  its  components  hydrogen  and 
oxygen,  and  in  the  gases  issuing  from  low  temperature 
fumaroles,  such  as  are  found  in  the  craters  of  volcanoes 
during  the  periods  between  eruptions  or  at  more  distant 
points  during  an  eruption,  the  content  of  hydrogen  is  rela- 
tively small  or  even  nothing.  It  has  long  been  recognized, 
also,  that  just  as  hydrogen,  methane,  and  carbon  monoxide 
are  found  in  the  higher  temperature  emanations  from  near 
the  molten  lava,  carbon  dioxide  is  correspondingly  abundant 
in  the  more  distant  low  temperature  emanations. 

Water  when  heated  by  itself  is  properly  regarded  as  an 
exceedingly  stable  substance  and  suffers  less  than  2%  of 
dissociation  at  temperatures  even  up  to  2000°C.  Heated 
in  contact  with  igneous  rocks,  however,  at  temperatures  at 
or  above  red  heat,  it  undergoes  large  dissociation,  particu- 
larly, it  would  seem,  whenever  substances  are  present  to 
unite  with  the  nascent  oxygen  set  free.  Gautier  has  shown 
that  iron  silicates  when  heated  to  redness  in  a  current  of 
steam  yield  a  gas  which  contains  no  less  than  65%  of  hydro- 
gen. It  is  doubtless  because  of  ready  oxidation  by  the 
nascent  oxygen  thus  set  free  by  dissociation,  that  oxygen 


THE  VAPORS  AND  GASES  OF  LAVA  75 

gas  is  one  of  the  least  abundant  of  those  found  in  volcanic 
emanations,  and  that  methane  and  hydrogen  sulphide  are 
also  sometimes  present  to  indicate  a  strongly  reducing  con- 
dition within  the  gaseous  mixture. 

It  is  an  inheritance  from  the  Kant-Laplace  hypothesis 
and  the  consequent  conception  of  a  still  molten  interior 
for  the  earth,  that  the  gases  of  lavas,  with  the  exception  of 
water  vapor,  are  so  generally  believed  to  be  emitted  from 
the  molten  interior.  These  gases  are  assumed  to  have  been 
absorbed  with  the  condensation  of  the  planet  from  its  vapor- 
ous nebula.  The  great  Viennese  geologist,  Eduard  Suess, 
who  held  to  this  view,  wrote  of  it,  "Just  as  molten  iron 
absorbs  extremely  large  quantities  of  gas,  and  gives  them 
off  again  as  it  cools,  so  the  globe  of  the  earth  once  absorbed 
extremely  large  quantities  of  gas,  which  it  is  now  still  con- 
tinuing to  emit."  He  says  further,  "Just  as  there  is  juvenile 
hydrogen,  so  there  is  juvenile  chlorine,  fluorine,  arsenic, 
carbon,  and  a  series  of  other  elements,  all  emitted  by  vol- 
canoes." The  name  "juvenile"  is  still  widely  used  for  such 
gases  as  are  assumed  to  be  derived  from  the  original  earth 
interior,  though  it  must  be  obvious  that  after  discarding  the 
notion  of  a  molten  earth  core,  not  only  the  question  of  the 
origin  of  these  gases  but  that  of  their  liberation  must  be 
taken  up  de  novo. 

The  large  volume  of  water  vapor  given  off  at  volcanic 
vents,  was  for  a  long  time  explained  by  assuming  that  ocean 
water  in  some  way  finds  a  passage  to  the  molten  magma 
in  the  subterranean  reservoir,  and  the  localization  of  vol- 
canic vents  either  upon  islands  of  the  sea  or  close  to  its 
borders  has  seemed  to  lend  color  to  this  view.  As  we  are 
to  see,  this  arrangement  of  the  vents  finds  another  explana- 
tion more  in  harmony  with  our  present  knowledge  of  the 
earth. 

It  must  now  be  our  purpose  to  see  whether  the  gases  and 
vapors  characteristic  of  lava  emanations  or  occluded  in  con- 
solidated igneous  rock,  are  satisfactorily  accounted  for  on 


76        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

the  theory  of  the  origin  of  lava  in  the  local  fusion  of  shale 
to  form  a  pocket,  and  the  ascent  of  the  resulting  magma 
to  the  surface  through  the  superincumbent  rock  cover. 

Omitting  the  eight  principal  constituents  of  shale  which 
have  already  been  considered  in  their  close  relation  to  the 
corresponding  constituents  in  the  average  igneous  rock, 
there  still  remain  a  number  of  others  present  in  less  amount 
but  yet  of  large  significance.  In  the  analysis  made  from 
a  composite  of  78  shales,  an  analysis  carried  out  in  the 
laboratory  of  the  United  States  Geological  Survey,  the  fol- 
lowing proportions  of  these  accessory  constituents  were 
obtained : 

Water  at  110°C 1.34 

Water  above   110°C 3.68 

Titanium  Dioxide    0.65 

Carbon  Dioxide   2.64 

Phosphorus    Pentoxide    0.17 

Sulphur  Trioxide   0.65 

Manganous  Oxide    Trace 

Barium  Oxide   0.05 

Carbon    (organic  origin) .  0.81 

Total 9.99 

It  is  not  necessary  to  assume  that  the  sulphur  was  present 
in  the  shale  in  the  form  of  the  oxide,  since  it  is  far  more 
probable  that  this  is  present  in  combination  with  iron  as 
pyrite,  iron  disulphide. 

It  has  already  been  pointed  out  that  when  in  contact 
with  rock  at  a  temperature  even  as  low  as  red  heat,  water 
is  to  a  very  large  extent  (65%)  dissociated  into  its  gaseous 
components,  hydrogen  and  oxygen.  The  carbon  dioxide  at 
higher  temperatures  dissociates  into  carbon  monoxide  and 
oxygen.  Oxygen  from  either  of  these  sources  in  its  nascent 
condition  will  readily  unite  as  opportunity  arises  with  the 
organic  carbon,  with  sulphur,  and  with  any  sodium  which 
has  been  set  free  from  sodium  chloride. 

In  its  ascent  toward  the  surface  with  the  magma,  the 
volume  of  CO  and  C02  will  be  augmented  by  calcination 


THE  VAPORS  AND  GASES  OF  LAVA  77 

of  carbonates  in  any  calcareous  sediments  which  are  assimi- 
lated, and  this  will  take  place  particularly  under  the  re- 
duced pressures  within  the  last  one  thousand  feet  before 
reaching  the  earth's  surface.  Chlorine  will  come  into  the 
gases  through  dissociation  of  any  sodium  chloride  encoun- 
tered by  the  magma  en  route,  and  the  sulphur  will  be 
augmented  by  the  breakdown  of  salts  of  sulphur  such  as 
are  admixed  with  sodium  chloride  in  sea  and  desert  salts. 
It  has  already  been  pointed  out  in  an  earlier  chapter  that 
a  comparison  of  the  composition  of  rocks  which  are  of  rela- 
tively deep-seated  consolidation  within  the  earth's  outer 
shell,  with  those  which  arrived  near  to  or  at  the  surface, 
shows  apparently  in  the  latter  group  an  increase  of  the 
bases  oxide  of  lime  and  oxide  of  sodium,  which  would 
result  from  such  assimilation  of  carbonates  and  chlorides 
as  has  been  described.  The  combination  of  free  oxygen 
with  sulphur,  carbon,  sodium  and  other  elements  probably 
explains  why  oxygen  is  among  the  rarest  of  the  gases 
emitted  from  lavas  and  why  these  have  so  generally  a  re- 
ducing character.  Few  geologists  have,  however,  been 
willing  to  assume  that  any  considerable  assimilation  of 
country  rock  goes  on  in  connection  with  the  movement  of 
magma,  Loewinson-Lessing,  Daly  and  Sederholm  being  ex- 
ceptions to  the  rather  general  rule. 

It  is  pertinent  now  to  inquire  what  the  observations  of 
geologists  tell  us  concerning  the  manner  of  opening  up  a 
conduit  through  which  magma  contained  in  a  subterranean 
chamber  may  be  conducted  to  the  earth's  surface.  It  was 
the  belief  of  Suess  that  in  few  cases  does  this  take  place 
through  a  fissure,  but  rather  after  the  manner  described  by 
Daly,  in  which  the  gases  issuing  from  the  magma  near  its 
upper  surface  are  there  permitted  to  unite  and  so  develop 
heat  after  the  manner  of  a  blowpipe.  He  says,  "Owing  to 
their  high  temperature  they  melt  and  stope  their  way 
through  the  overlying  rocks,  and  thus  force  their  way  up- 


78        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

wards."  Elsewhere  he  has  described  the  process  as  not  un- 
like that  by  which  a  red-hot  iron  makes  its  way  upward 
through  a  board. 

Barrell  and  Daly  have  developed  this  process  of  overhead 
stoping  in  special  studies,  and  the  parallel  is  drawn  with  the 
quarrying  process  of  that  name  in  use  by  miners.  The 
hot  gases  fuse  and  assimilate  portions  of  the  roof-rock  near 
pre-existing  fissures  and  eventually  quarry  away  blocks 
which,  once  detached  from  the  roof,  tend  to  founder  in 
the  magma. 

The  source  of  the  nitrogen  which  is  generally  sparingly 
present  in  magmas,  though  sometimes  in  large  amount,  has 
generally  been  attributed  to  admixture  of  atmospheric  air. 
In  cases  where  air  is  excluded,  its  origin  must  be  sought  in 
the  air  which  is  held  in  solution  in  the  underground  water 
of  surface  origin  and  involved  in  the  lava  during  its  rise  to 
the  surface.  Such  nitrogen  is  usually  found  mixed  with 
argon.  Quite  surprising  was  it  therefore  to  find  in  the 
gases  withdrawn  from  the  lava  lake  of  Kilauea,  as  first  re- 
ported by  Day  and  Shepherd,  that  analysis  showed  nitrogen 
quite  free  from  argon  comprising  from  11%  to  63%  of  the 
whole.  Later  studies  yielded  quite  different  results  and 
ones  much  more  easily  explained.  Argon  is  present  in  all 
samples  which  were  tested  and  the  nitrogen  was  found  to 
range  from  less  than  1%  to  more  than  23%.  Professor  Jag- 
gar,  whose  studies  carried  out  through  a  decade  of  pains- 
taking observations  from  his  station  at  Kilauea  have  yielded 
such  rich  results,  has  shown  that  air  is  drawn  into  the  lava 
column  of  Halemaumau  as  though  in  a  furnace,  and  that 
it  plays  a  large  role  in  supplying  the  energy  of  the  eruptive 
forces. 

Those  gases  which  are  not  expelled  from  the  lava  at  the 
time  of  its  eruption  are  locked  up  in  the  congealed  mass. 
These  are  liberated  when  the  rock  is  heated  sufficiently. 

The  many  studies  of  gases  which  are  driven  off  at  high 
temperatures  from  igneous  rocks,  and  notably  those  which 


THE  VAPORS  AND  GASES  OF  LAVA  79 

were  made  by  R.  T.  Chamberlin,  are  quite  striking  in 
their  similarity  to  those  liberated  by  similar  heating  of 
sedimentary  rocks  and  with  many  which  are  wholly  uncon- 
nected with  volcanoes.  This  has  led  to  the  conviction  that 
we  are  here  dealing  with  materials  absorbed  from  the 
atmosphere  and  found  in  a  condition  dependent  chiefly  on 
the  process  of  expulsion. 

In  summarizing,  then,  we  see  that  the  principal  gases  of 
lava — water  (gas  or  vapor)  hydrogen,  carbon  dioxide,  carbon 
monoxide,  hydrochloric  acid,  chlorine,  sulphur  dioxide,  and 
oxygen — are  all  to  be  accounted  for  either  by  the  materials 
already  present  in  shale  which  by  fusing  may  be  considered 
to  have  produced  the  lava,  or  by  accessions — largely 
chlorine  and  sulphur — which  have  been  secured  during  the 
magma's  ascent  to  the  earth's  surface.  It  is  no  doubt  signifi- 
cant of  this  manner  of  origin  for  chlorine  and  sulphur,  that 
of  all  the  gases  of  lavas  chlorine  and  sulphur  in  their  dif- 
ferent-states of  combination  are  the  least  constant  in  quan- 
tity, both  being  frequently  absent  altogether  though  at 
other  times  present  in  large  volume.  On  Mt.  Vesuvius  dur- 
ing the  waning  stages  of  the  great  eruption  of  1906,  Ferret 
detected  both  hydrochloric  and  sulphuric  acid.  It  has 
sometimes  occurred  after  an  eruption  of  Vesuvius  that  the 
solution  of  hydrochloric  acid  in  rain-water  has  been  of 
such  strength  as  to  destroy  the  vineyards  upon  the  slopes. 
Sodium  chloride  in  incrustations  and  in  various  sublima- 
tions are  found  in  the  lava,  and  the  iron  in  the  basalt  is 
often  responsible  for  a  yellow  tinge  through  the  formation 
of  a  surface  coat  of  ferrous  chloride  which  the  uninitiated 
believe  to  be  sulphur. 

Worcester  in  his  report  on  the  eruption  of  Taal  in  the 
Philippines  on  the  30th  of  January,  1911,  described  a  heavy 
precipitation  of  mud  so  strongly  charged  with  sulphuric 
acid  that  the  bodies  of  the  natives  who  perished  in  it  were 
found  flayed  by  the  acid. 


80        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


LITERATURE 

F.  LOEWINSON-LESSING.  Studien  ueber  die  Eruptivgesteine,  C.  R.  VII 
Sess.  Cong.  Geol.  Intern.,  St.  Petersburg,  1897,  1899,  III,  Die  Magma- 
tische  Differentiation,  pp.  366-370. 

R.  T.  CHAMBERLIN.  The  gases  in  rocks,  Doctor's  dissertation  Univ.  of 
Chicago,  Lippincott,  1908,  p.  80. 

F.  A.  FERRET.  Vesuvius,  characteristics  and  phenomena  of  the  repose- 
period,  Am.  Jour.  Sci.,  vol.  28,  1909,  pp.  422-423. 

DEAN  C.  WORCESTER.  Taal  volcano  and  its  recent  destructive  eruption, 
Nat.  Geogr.  Mag.,  vol.  23,  1912,  pp.  350-355. 

A.  BRUN.  Recherches  sur  Pexhalaison  volcanique,  Geneva  and  Paris,  1911, 
p.  277,  pis.  34. 

WILLIAM  H.  HOBBS.  Some  considerations  concerning  the  place  and  the 
origin  of  lava  maculae,  Gerlands  Beitrage  zur  Geophysik,  vol.  12,  1913, 
pp.  330-361. 

A.  L.  DAY  and  E.  S.  SHEPHERD.  Water  and  volcanic  activity,  Bull.  Geol. 
Soc.  Am.,  vol.  24,  1913,  pp.  573-606,  pis.  34. 

E.  S.  SHEPHERD.    The  composition  of  the   gases  of  Kilauea,  Bull.  Ha- 

waiian Volcano  Observatory,  vol.  7,  no.  7,  July,  1919,  pp.  1-4. 
EDUARD  SUESS.    The  face  of  the  earth,  Translation  by  Sollas,  vol.  4,  1909, 

pp.  548-559. 
R.  A.  DALY.     The   mechanics   of  igneous  intrusion,  Am.  Jour.   Sci.    (4) 

vol.  15,  1903,  pp.  269-278. 
JOSEPH   BARRELL.    Geology  of  the  Marysville  mining  district,   Montana, 

Prof.  Pap.  no.  57,  U.  S.  Geol.  Surv.,  1907,  pp.  151-178. 
J.  P.  IDDINGS.    The  problem  of  volcanism,  Yale  University  Press,  1914, 

pp.  7-15. 

F.  W.  CLARKE.    The  data  of  geochemistry,  Bull.  616,  U.  S.  Geol.  Surv., 

1920,  chap.  8,  pp.  255-285. 

J.  J.  SEDERHOLM.  On  regional  granitization  (or  anatexis)  Comptes  Rendus 
Congres  Geol.  Intern.  12me  Sers.  Canada,  1913,  p.  319. 

PENTTI  ESCOLA.  On  volcanic  necks  in  Lake  Janisjarvi  in  Eastern  Fin- 
land, Bull,  de  la  Comm.  Geol.  de  Finlande,  no.  55,  1921,  pp.  1-13. 

T.  J.  JAGGAR,  JR.  Volcanologic  Investigations  at  Kilauea,  Am.  Jour.  Sci. 
(4),  vol.  44,  1917,  pp.  161-220. 


CHAPTER  VII 

THE  CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS 
PASSED  THROUGH 

IT  is  only  to  the  person  looking  out  over  the  sea,  or  upon 
a  vast  plain,  that  an  impression  of  the  earth's  rotundity 
is  received  through  the  senses,  and  even  under  such  circum- 
stances it  is  only  the  discerning  observer  who  gains  any  other 
impression  than  that  of  monotonous  flatness.  The  eclipsing 
of  the  lower  portion  of  distant  objects  seen  over  the  bulging 
surface  of  the  sea,  especially  ships  "hull  down"  on  the  hori- 
zon, is  apt  to  give  us  our  earliest  clear  impression  of  earth 
sphericity,  though  an  even  better  demonstration  is  possible 
at  such  times  as  the  profile  of  the  earth  is  shadowed  upon 
the  moon.  This  latter  demonstration  makes  clear  that  if 
the  earth's  figure  departs  at  all  from  a  perfect  sphere,  the 
variation  must  be  by  extremely  small  amounts. 

Precise  comparative  measurements  of  the  length  of  a 
degree  of  latitude  along  a  meridian,  made  in  the  equatorial 
and  near  polar  sectors  of  the  earth,  have  nevertheless  shown 
that  the  figure  to  which  the  lithosphere  and  its  watery 
envelope  approach  is  an  ellipsoid  of  revolution  or  spheroid, 
whose  polar  diameter  is  shorter  than  its  equatorial  diameter 
by  about  l/299th,  a  difference  far  too  small  to  be  appre- 
ciable to  the  eye  upon  any  demonstration  model  having  the 
same  proportions.  This  flattening  of  the  spheroid  of  the 
earth  about  its  poles,  which  constitutes  its  oblateness, 
though  small,  must  not  for  that  reason  be  accounted  as  un- 
important. As  a  matter  of  fact  it  is  of  the  utmost  signifi- 
cance as  indicating  that  the  earth's  rotational  velocity  was 
once  much  greater  than  it  is  today,  for  such  greater  velocity 
would  have  increased  proportionately  the  centrifugal  force 

81 


82        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

and  induced  bulging  in  the  equatorial  region  with  corre- 
sponding contraction  at  the  ends  of  the  axis  of  rotation. 

The  departure  of  the  earth's  figure  from  that  of  a  perfect 
sphere  toward  an  oblate  spheroid,  is  not  the  only  one  which 
it  has  undergone.  Of  the  same  order,  and  of  very  nearly 
the  same  amount,  are  other  departures  from  the  spherical 
figure  which  are  revealed  by  the  size,  shape  and  positions 
of  the  ocean  basins  on  the  one  hand,  and  the  continental 
platforms  upon  the  other.  The  bottom  of  the  greatest  deep 
of  the  sea  is  about  twelve  miles  nearer  the  earth's  center 


FIG.  27. — A  tetrahedron  with  bulging  faces  and  truncated  angles  to  bring 
out  its  contrasted  antipodal  relationships 

than  the  crest  of  Mt.  Everest,  the  highest  elevation  upon 
the  continents,  and  the  geographic  poles  of  the  earth  as  a 
consequence  of  oblateness  are  on  the  average  thirteen  and 
one-half  miles  nearer  the  earth's  center  than  the  average 
of  points  on  the  equator. 

The  figure  toward  which  the  earth  departs  from  the 
spheroid,  as  indicated  by  the  distribution  of  land  and  sea, 
has  often  been  described  as  a  tetrahedron,  though  the  state- 
ment is  almost  grotesque  when  put  in  this  blunt  form.  The 
tetrahedron  is  a  solid  body  which  has  four  equal  plane  sur- 
faces, each  of  which  is  an  equilateral  triangle,  and  four 


CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS  PASSED       83 

"solid"  angles  in  each  of  which  three  edges  or  dihedral 
angles  meet.  Such  a  solid,  or  polyhedron,  has  obviously 
so  little  resemblance  to  the  spheroid  that  to  show  the  rela- 
tionship it  is  far  better  to  compare  a  modified  tetrahedron 
with  bulging  faces  and  deeply  truncated  angles.  When  this 
is  done  (Fig.  27),  we  see  that  each  face  of  the  model  repre- 
sents a  portion  of  the  surface  which  comes  nearest  to  the 
center  of  the  figure,  while  opposite  to  such  a  face  there  is 
always  a  solid  angle  representing  the  part  of  the  figure 
farthest  away  from  its  center  of  form.  The  edges  in  which 


Angara 


FIG.  28.— The  earth 


as  a  departure  of  the  spheroid  toward  the 
tetrahedron 


the  bulging  faces  meet,  represent  those  parts  of  the  surface 
which  are  farthest  away  from  the  center,  excepting  only 
the  solid  angles  themselves  in  which  these  edges  are  united. 
If  now  we  stand  the  model  upon  one  of  its  truncated  solid 
angles,  so  as  to  represent  the  great  protuberance  of  the 
Antarctic  continent,  the  opposite  (top)  face  of  the  model 
will  serve  to  represent  the  deep  oceanic  basin  of  the  Arctic 
Ocean,  with  its  encircling  rim  of  land  in  at  least  partial 
correspondence  with  the  tetrahedral  edges  bounding  this 
face  of  the  figure  (Fig.  28).  The  three  remaining  truncated 
solid  angles  all  lie  within  this  land  rim  and  fall  into  rough 


84        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

correspondence  with  three  ancient  land  areas  of  the  globe 
which  have  been  characterized  as  its  "shields"  or  "coigns." 
These  coigns  are  respectively  an  area  surrounding  Hud- 
son's Bay  known  as  the  "Laurentian  or  Canadian  Shield," 
one  surrounding  the  Baltic  Sea  called  the  "Baltic  Shield," 
and  a  third  in  east-central  Siberia  referred  to  as  the  "Angara 
Shield."  These  oldest  known  land  areas  of  our  planet  con- 
stitute the  nuclei,  to  which  later  marginal  land  zones  have 
been  added  in  a  very  complex  historical  sequence  to  produce 
the  present  continents. 

There  are  several  tests  which  may  be  applied  to  the  litho- 
sphere  to  determine  whether  it  may  properly  be  regarded 
as  departing  in  a  significant,  even  if  not  in  a  large,  measure 
from  the  ideal  figure  of  the  spheroid  toward  the  modified 
tetrahedron.  If  we  include  in  the  continental  platforms,  as 
is  customary,  the  shallow  sea.-floor  borders  of  the  con- 
tinental shelves,  these  platforms  include  about  one-third 
of  the  entire  superficial  area  of  the  lithosphere,  as  against 
about  one-half  of  the  area  which  is  included  in  the  rela- 
tively deep  sea-floor  of  the  oceanic  platform,  the  remaining 
portion  being  largely  comprised  within  the  continental 
slopes  joining  the  two  platforms.  In  each  medial  line  of 
our  tetrahedral  figure  (Fig.  27),  the  essentially  antipodal 
contrasts  characteristic  of  the  figure  are  displayed.  Thus, 
a,  represents  a  short  radius  beneath  a  face  of  the  figure, 
and  b,  a  long  radius  beneath  the  opposite  angle.  For  the 
lithosphere  this  antipodal  relationship  is  well  brought  out 
by  the  distribution  of  land  and  water,  or  better  by  the  dis- 
tribution of  the  continental  and  ocean  platforms.  The  con- 
trast is  greatest  along  the  polar  diameter  where  the  basin  of 
the  Arctic  is  opposed  to  the  high  continent  of  the  Antarctic. 
The  other  coigns  of  the  earth  are  only  less  strikingly  op- 
posed to  the  ocean  basins,  and  just  as  we  have  the  run  of 
edges  to  the  tetrahedral  figure  represented  by  the  circuit 
of  land  about  the  Arctic,  so  we  have  the  continuous  zone 
of  oceans,  the  so-called  "south  seas,"  to  represent  the  taper- 


CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS  PASSED       85 

ing  figure  of  the  tetrahedron  about  its  southern  apex.  In 
general  it  is  true  that  if  the  finger  be  placed  upon  any  por- 
tion of  the  continental  platform  as  represented  upon  a 
correct  model,  a  portion  of  the  oceanic  platform  is  found 
in  the  antipodal  position.  Corresponding  to  the  three  faces 
of  the  tetrahedron  which  converge  toward  the  southern 
apex,  upon  the  lithosphere  we  find  the  Atlantic,  Pacific,  and 
Indian  oceans  in  roughly  these  positions,  and  all  these 
oceans  broaden  progressively  in  the  southerly  direction  until 
they  unite. 

Inasmuch  as  the  force  of  gravity  at  any  point  on  the 
earth's  surface  has  a  value  inversely  proportional  to  the 
distance  through  which  it  acts,  with  the  earth's  mass  con- 
centrated at  its  center  of  form,  the  average  value  for  the 
force  of  gravity  for  points  within  the  southern  hemisphere 
should  be  less  than  the  average  value  within  the  northern 
hemisphere,  the  Arctic  basin  excepted.  This  is  a  well  at- 
tested fact  demonstrated  by  investigations  carried  out  with 
pendulums.  The  period  of  vibration  of  the  "seconds" 
pendulum  is  a  function  of  the  local  value  of  gravity,  and 
when  pendulum  clocks  are  transported  from  the  northern 
to  the  southern  hemisphere,  the  length  of  the  pendulum 
must  be  adjusted  if  they  are  to  keep  accurate  time. 

We  see,  therefore,  that  however  grotesque  may  appear 
the  bald  statement  that  the  earth  departs  in  its  figure  from 
a  spheroid  toward  a  tetrahedron,  such  an  assertion  ex- 
presses observed  fact  and  not  theory  only.  The  absurdity 
would  be  to  state  that  its  figure  is  that  of  a  tetrahedron. 
Its  spheroidal  form  we  can  easily  and  naturally  account  for 
by  its  rotation  when  in  a  more  plastic  condition  than  it  is 
today  and  rotating  more  rapidly. 

All  the  planets  are  similarly  spheroidal  in  figure,  with 
varying  degrees  of  oblateness  which  are  believed  to  express 
differences  in  original  rotational  velocities.  The  degree  in 
which  oblateness  expresses  velocity  of  rotation  may  be 
visibly  demonstrated  by  fixing  a  wooden  hoop  at  one  point 


86        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

of  its  rim  in  the  vertical  axis  of  a  revolving  table  and 
allowing  the  stiff  rod  which  forms  the  rotating  axis  to  pass 
loosely  through  an  aperture  at  the  opposite  point  of  the  rim, 
so  as  to  keep  the  position  of  the  hoop  vertical  during  its 
rotation  though  permitting  it  to  move  the  free  side  of  its 
ring  upward  or  downward  in  obedience  to  the  varying 
centrifugal  force.  During  rapid  rotation  the  hoop  appears 
as  a  spheroid  and  as  the  table  rotates  more  and  more 
rapidly  the  oblateness  of  the  spheroid  increases,  only  to  fall 
off  correspondingly  as  the  velocity  is  diminished. 

But  what  is  the  explanation  of  the  departure  of  the 
earth's  spheroid  toward  a  tetrahedral  figure?  The  natural 
explanation  is  found,  and  so  far  as  known  the  only  one  that 
has  been  offered,  in  a  continued  reduction  of  volume  of  the 
lithosphere  which  is  believed  to  be  the  result  of  the  earth's 
secular  cooling — loss  of  internal  heat  to  the  surrounding 
space — as  has  already  been  pointed  out  in  the  previous 
chapter.  The  sphere  to  which  the  earth's  spheroid  so  closely 
approximates  is  of  all  regular  figures  the  one  which  has  the 
smallest  superficial  area  for  a  given  volume.  Now  the  outer 
shell  of  the  lithosphere  must  early  have  acquired  an  essen- 
tially stable  temperature,  whereas  the  interior  portion,  pro- 
tected by  its  poorly  conducting  shell,  lagged  behind  hi  the 
cooling  process,  and  has  apparently  continued  to  lose  heat 
throughout  the  geological  ages  continuing  to  the  present 
time.  The  shell  having  thus  acquired  a  fixed  superficial 
area,  has  been  compelled  to  undergo  changes  of  figure  in 
closing  in  upon  its  ever  smaller  core  under  the  impelling 
force  of  gravitation. 

Now  it  is  true  that  regular  geometric  figures  have  smaller 
volume  for  any  given  surface  in  proportion  as  they  have  few 
plane  surfaces  or  faces,  and  the  tetrahedron  with  its  four 
faces  represents  the  lower  limit  to  this  series,  as  the  sphere 
with  its  infinite  number  of  faces  does  the  upper,  for  the 
reason  that  a  lesser  number  of  plane  surfaces  than  four 
could  not  be  made  to  enclose  space.  The  solid  series  repre- 


CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS  PASSED        87 

sen  ted  in  sequence  in  fig.  29  from  the  sphere  through  bodies 
having  respectively  48,  24,  12,  and  8  faces  to  the  limiting 
tetrahedron  of  4  faces,  will  help  to  make  this  clear,  though 
it  must  on  no  account  be  assumed  that  the  earth  has  passed 
to  its  present  tetrahedral  tendency  by  the  route  represented. 
Experiments  carried  out  with  hollow  spheres  made  from 
thin  and  flexible  metal  have  demonstrated  that  these  bodies 
deform  toward  a  tetrahedron  when  reduction  of  their  vol- 
ume is  brought  about  through  exhaustion  of  the  contained 
air.  It  is  therefore  no  more  unnatural  to  account  for  the 
departure  of  the  earth's  figure  toward  a  tetrahedron  as  a 
result  of  reduction  of  volume  through  secular  cooling,  than 
it  is  to  explain  the  departure  of  the  sphere  toward  the 
spheroidal  figure  by  almost  exactly  the  same  amount  due 


FIG.  29. — Series  of  polyhedrons  from  the  sphere  with  greatest  volume  to 
the  tetrahedron  with  least  volume 

to  an  earlier  higher  temperature  and  higher  rotational 
velocity. 

We  might,  perhaps,  carry  the  subject  of  departures  of 
the  geoid  from  the  spheroid  of  tetrahedral  tendency,  so  as 
to  take  account  of  the  fact  that  the  disproportionate  size 
of  the  Pacific  involves  a  certain  flattening  of  the  spheroid 
in  one  plane  parallel  to  its  polar  axis  and  give  it  somewhat 
the  form  of  a  potato,  but  the  cause  of  this  departure  is 
still  obscure,  at  least  to  the  geologist. 

It  is  not,  of  course,  to  be  assumed  that  the  earth  spheroid, 
or  geoid,  underwent  transformation  toward  the  tetrahedron 
through  any  such  series  of  figures  as  are  represented  hi  the 
progression  of  Fig.  29.  To  learn  in  outline  what  the  inter- 
mediate stages  of  earth  physiognomy  have  been,  we  must 
consult  the  past  history  of  the  earth  as  revealed  in  the  rocks 
themselves,  the  essential  documents  of  geological  history. 


88        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

Geological  thought,  and  indeed  especially  American  geo- 
logical thought,  has  in  respect  to  theories  of  continental 
evolution  undergone  great  transformations  within  recent 
years.  Throughout  the  first  third  of  the  nineteenth  cen- 
tury the  ideas  of  the  so-called  "catastrophists"  dominated 
the  field,  their  view  being  that  geological  history  had  been 
punctuated  by  great  catastrophes  in  which  a  seemingly 
quiet  condition  of  earth  forces  permitting  plant  and  animal 
life,  had  been  suddenly  interrupted  by  a  world  cataclysm 
in  which  great  surface  changes  were  brought  about  and 
all  forms  of  life  destroyed,  after  which  plants  and  animals 
of  new  forms  were  created.  The  period  in  which  these  ideas 
prevailed  was  one  in  which  the  Church  dominated  scientific 
thought,  and  a  literal  interpretation  of  the  first  chapter  of 
Genesis  formed  the  basis  of  earth  history.  In  all  Christian 
countries  geologists  labored  diligently  to  bring  their  ob- 
servations into  literal  harmony  with  the  Scriptures,  and 
those  who  dared  openly  to  express  opposing  views  suffered 
persecution  for  their  independence. 

The  appearance  in  the  thirties  of  the  last  century  of  that 
masterpiece  of  geological  writing,  Sir  Charles  LyelFs  Prin- 
ciples of  Geology,  marked  the  opening  of  a  new  era  in  which 
was  brought  about  an  entire  transformation  of  geological 
thought.  The  fundamentally  new  conception  at  the  basis 
of  Lyell's  work  was  that  the  cataclysm  which  had  been 
supposed  to  end  the  geological  periods  with  destruction  of 
life  and  creation  of  new  forms,  had  no  existence;  but  that 
the  conditions  of  the  present  are  a  key  to  the  past  as  well. 
Such  a  change  involved  the  principle  of  evolution  of  life  as 
opposed  to  the  idea  of  relatively  sudden  recreations  of  life 
following  upon  its  destruction,  and  this  new  trend  in  geo- 
logical thought  antedated  by  some  years  the  great  period 
of  change  in  the  biological  field  marked  by  the  appearance 
of  Darwin's  Origin  of  Species. 

It  was  only  natural  that  the  pendulum  of  thought,  swing- 
ing away  from  a  revealed  error,  should  be  carried  past  the 


CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS  PASSED       89 

correct  norm  to  the  very  opposite  side  of  the  arc  as  a  result 
of  the  rather  extravagant  claims  for  the  slowness  of  geo- 
logical processes,  as  opposed  to  the  long  approved  sudden- 
ness of  geological  change.  It  will  be  pointed  out  in  the 
succeeding  chapter  that  different  sections  of  the  earth's  sur- 
face now  differ  most  widely  in  the  rate  at  which  geological 
change  is  taking  place,  and  that  in  the  British  Isles  such 
change  is  today  very  near  to  the  minimum.  It  is  there- 
fore in  no  way  surprising  that  Sir  Charles  Lyell,  and  his 
followers  even  to  the  present  day,  have  continued  to  make 
drafts  upon  the  bank  of  time  for  which  to  the  writer  there 
seems  to  be  little  warrant. 

The  idea  of  a  minimum  of  physical  change  in  combina- 
tion with  a  prodigious  length  of  geological  time  was  bol- 
stered in  America  by  the  foremost  American  geologist  of 
his  time,  Professor  James  D.  Dana,  who  set  up  a  theory 
that  the  ocean  basins  throughout  geological  history  have 
maintained  essentially  the  positions  they  have  today,  a 
doctrine  widely  known  and  accepted  as  a  fundamental  tenet 
of  the  science,  under  the  name  "permanence  of  the  ocean 
basins." 

It  was  not  until  a  half  century  after  the  appearance  of 
LyelTs  masterpiece,  which  is  generally  regarded  as  giving 
date  to  the  modern  era  in  geological  thinking,  that  the  ap- 
pearance of  a  work  of  the  same  dominating  quality  marked 
the  beginning  of  the  returning  swing  of  the  pendulum,  not 
indeed  back  to  the  ideas  of  the  catastrophists,  but  away 
from  that  of  slow  and  moderate  geological  changes  in  the 
earth's  surface,  which  had  been  the  essential  element  in 
Lyell's  "uniformitarianism."  This  masterpiece  was  entitled 
Das  Antlitz  der  Erde,  The  Face  of  the  Earth,  and  was  the 
work  of  Professor  Eduard  Suess  of  Vienna.  In  it  he  brought 
out  arguments  for  the  most  profound  transformations  of 
the  ocean  basins  and  the  continental  platforms.  Against 
the  idea  that  the  positions  originally  taken  by  the  ocean 
basins  had  been  maintained  throughout  geological  history, 


90        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

it  was  shown  on  the  basis  of  the  now  known  development 
and  distribution  of  life,  faunal  and  floral  geography,  that 
across  where  today  are  great  ocean  basins  there  once 
stretched  vast  continents  larger  than  any  that  exist  today. 
The  second  largest  of  our  oceans,  the  Atlantic,  and  the 
third  largest,  the  Indian,  were  in  middle  geological  time  in 
large  part  continents.  Professor  Suess's  broad  generaliza- 
tion concerning  the  main  lines  in  earth  physiognomy  of 
the  past  as  well  as  the  present,  will  continue  to  stand  out 
among  the  landmarks  in  the  history  of  geological  science. 


Ar  £f*o  or  Eozo/c  E*A 

FIG.  30. — Generalized  expression  of  the  earth's  figure  at  the  close  of  the 
first  great  era  of  geological  history 

In  the  field  of  the  biological  sciences,  the  modifications 
of  Darwin's  theory  in  the  direction  of  recognition  of  muta- 
tions, sudden  changes  in  the  evolution  of  species,  has  like- 
wise tended  to  the  ascribing  of  somewhat  less  importance 
to  the  earlier  view  concerning  the  extreme  slowness  of  de- 
velopment of  the  new  species  of  plants  and  animals. 

With  this  general  introduction  we  shall  now  pass  to  a 
discussion  of  the  outline  of  evolution  in  earth  physiognomy 
in  an  attempt  to  determine  through  what  transformations 
the  spheroid  of  earlier  planetary  history  has  passed  in  ar- 
riving at  its  present  tetrahedral  leanings.  Attention  has 


CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS  PASSED       91 

already  been  drawn  to  the  three  ancient  continents  of  the 
northern  hemisphere  which  have  been  described  as  coigns 
or  shields — the  Laurentian,  Baltic,  and  Angara  shields.  In 
their  earlier  history  they  existed  without  the  enveloping 
portions  of  the  present-day  continental  platforms.  Some- 
what less  clearly  defined,  perhaps  in  part  because  of  the 
less  advanced  state  of  geological  knowledge  for  the  southern 
hemisphere,  three  ancient  shields  have  been  recognized  to 
the  southward  of  the  equator,  the  Brazilian,  African  and 
Australian  shields.  In  a  broad  way  these  southern  shields 
took  position  similar  to  the  northern  ones  in  being  some- 


FIG.  31. — The  continents  at  the  end  of  the  Paleozoic  era  (after  Arldt). 

what  evenly  spaced  in  respect  to  longitude  and  in  rough 
alignment  as  respects  the  northern  and  southern  coigns. 
Thus  at  the  conclusion  of  the  first  great  geological  era,  the 
Eozoic,  the  earth  physiognomy  was,  we  have  reason  to  be- 
lieve, comparatively  simple  and  somewhat  as  represented 
by  figure  30. 

Subsequent  to  the  Eozoic  era  the  coigns  became  enlarged 
by  marginal  growth,  and  after  a  series  of  changes  which 
cannot  be  here  followed  in  detail,  there  came  about  a  dis- 
tribution of  land  and  water  which  was  in  striking  contrast 
both  with  that  of  the  earlier  era  and  with  that  of  the  present 
time. 


92        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

Today  the  continents  and  the  oceans  may  be  described  as 
elongated  on  north  and  south  lines,  if  we  have  regard  to 
the  Caspian  depression  as  an  interrupted  continuation  of 
the  basin  of  the  Indian  Ocean.  Quite  otherwise  was  the  dis- 
tribution at  the  close  of  the  second  great  era  of  geological 
history,  the  Paleozoic,  when  the  distribution  was  more  on 
east  and  west  lines.  In  the  southern  hemisphere  a  great  con- 
tinent which  has  been  called  Gondwana  Land,  stretched 
from  South  America  eastward  across  Africa  to  Australia 


AT  £/V0 

FIG.  32. — Generalized  expression  of  the  earth's  figure  at  the  end  of  the 
second  great  era  of  geological  history 

taking  in  the  intervening  ocean  basins.  Somewhat  less 
continuously,  apparently,  the  North  Atlantic  Ocean  was 
bridged  by  land  to  form  a  real,  as  opposed  to  the  mythical 
"Atlantis,"  which  may  be  referred  to  as  Appalachia- 
Armorica;  and  between  the  northern  and  southern  con- 
tinents lay  a  dividing  sea  which  has  been  named  by  Pro- 
fessor Suess  the  Sea  of  Tethys,  the  consort  of  oceans 
(Fig.  31). 

To  express  this  aspect  of  the  earth  physiognomy  as  an 
intermediate  stage  toward  the  present  tetrahedral  leanings, 
we  arrive  at  the  form  represented  in  figure  32.  Such  a 


CHANGES  OF  FIGURE  WHICH  THE  EARTH  HAS  PASSED       93 

figure  with  its  reentrant  equatorial  girdle  suggests  the  well 
known  twin  plane  of  a  compound  crystal,  and  it  has  there- 
fore been  denominated  the  twin  plane. 

During  the  next  geological  era,  the  Mesozoic,  there  came 
about  those  profound  changes  which  resulted  in  the 
foundering  of  all  parts  of  the  Gondwana  continent  except 
those  which  now  remain  in  South  America,  Africa,  Hindu- 


FIG.  33. — Generalized  expression  of  the  present  figure  of  the  earth 

stan  and  Australia,  Farther  north  the  portions  of  Appala- 
chia-Armorica,  which  separate  North  America  from  Europe, 
foundered  to  form  the  North  Atlantic  Ocean;  and  to  the 
eastward  of  the  Mediterranean  the  Mesozoic  geosyncline 
was  elevated  to  form  a  great  belt  of  mountains  so  lofty 
as  to  be  referred  to  as  the  "roof  of  the  world."  Thus  was 
ushered  in  the  earth  of  present-day  aspect,  the  physiognomy 
of  which  is  broadly  outlined  in  Fig.  33. 

LITERATURE 

SIR  CHARLES  LYELL.    Principles  of  geology,  2  vols.,  llth  ed.,  1872. 

JOHN  W.  JUDD.    The  coming  of  evolution,  Cambridge  manuals  of  science 

and  literature,  Camb.  Univ.  Press,  1910,  p.  171. 
EDUARD  SUESS.    The  face  of  the  earth,  English  translation  by  Sollas,  5  vols., 

Clarendon  Press,  1904. 


94        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

W.  LOWTHIAN  GREEN.  Vestiges  of  a  molten  globe,  as  exhibited  in  the 
figure  of  the  earth's  volcanic  action  and  physiography,  pt.  1,  London, 
1875;  part  2,  Honolulu,  1887. 

JOHN  W.  GREGORY.  The  plan  of  the  earth  and  its  causes,  Geogr.  Jour., 
vol.  13,  1899,  pp.  225-251. 

B.  K.  EMERSON.  The  tetrahedral  earth  and  the  zone  of  the  intercon- 
tinental seas,  Presidential  Address,  Bull.  Geol.  Soc.  Am.,  vol.  11, 
1900,  pp.  61-96,  pis.  9-14. 

THEODOR  ARLDT.  Die  Entwicklung  der  Kontinente  und  ihrer  Lebewelt, 
Leipzig,  1907,  p.  729,  pis  23. 

WILLIAM  H.  HOBBS.  Earth  features  and  their  meaning,  Macmillan,  1912, 
chap.  2,  The  figure  of  the  earth. 


CHAPTER  VIII 

THE   PRESENT   REGIONS   OF   RAPID   CHANGE 

WE  have  seen  that  the  earth's  figure  as  shown  by  the 
changing  distribution  of  land  and  sea  has  more  than  once 
undergone  profound  changes  during  the  geologic  past,  and 
that  in  the  transformation  from  one  figure  to  another  the 
strains  must  obviously  have  been  greatest  within  certain 
belts  or  zones  which  separate  the  sinking  from  the  rising 
sectors.  If  such  large-scale  deformations  have  not  alto- 
gether ceased,  a  question  of  absorbing  interest  is  concerned 
with  the  place  of  their  operation  today.  Now  movements 
of  the  earth's  surface  shell  do  not  take  place  uniformly 
through  a  slow  continuous  warping  only  of  the  shell,  as  was 
so  long  believed;  but  stresses  which  tend  to  produce  move- 
ment are  resisted  until  they  have  accumulated  sufficiently 
to  overcome  the  resistance,  whereupon  partial  relief  may 
be  obtained  through  a  jolting  displacement  and  subsequent 
readjustments,  which  successive  joltings  are  manifested  to 
us  as  earthquakes  and  their  "after-shocks/' 

In  the  distribution  of  earthquakes  is  to  be  found  the  locus 
of  most  pronounced  earth  movement  today — the  zones  of 
present  maximum  change  in  figure  of  the  straining  litho- 
sphere.  For  the  continental  areas  we  have  long  known 
the  position  of  the  earthquake  districts.  Such  visitations 
are  too  destructive  of  life  and  property  not  to  have  been 
mapped  out  on  the  basis  of  historical  data,  at  least  for  the 
more  thickly  settled  portions  of  the  earth.  As  assembled 
by  de  Montessus  de  Ballore,  these  are  given  in  the  Moll- 
weide  projection  hi  figure  34.  It  is  quite  as  important  that 
we  know  the  part  of  the  sea-floor  in  these  movements, 
hidden  though  it  is  beneath  the  hydrosphere.  It  is  only 

95 


96        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

during  the  present  generation  that  methods  have  been  per- 
fected for  exploring  the  bottom  of  the  sea  for  earthquake 
adjustments,  though  some  indication  of  the  more  mobile 
zones  are  supplied  us  in  the  location  of  earthquake  sea- 
waves,  long  erroneously  referred  to  as  "tidal  waves." 
Modern  seismographs,  or  earthquake-registering  instru- 
ments, set  up  in  a  coordinated  system  of  earthquake  stations 
properly  distributed  over  the  surface  of  the  planet,  now 
regularly  record  all  sudden  major  movements  of  the  litho- 


reos  (de  Montessusl  W  Cooafa  visited  by  Seumic  Sea  Waves  (  Rudolph) 

*''„  Geosynclines  of   Mesozoic    Era 

FIG.  34. — Map  on  Mollweide  projection  to  show  the  distribution  of  historic 
earthquakes  for  the  land  areas  of  the  globe  (after  de  Montessus) 

sphere  surface,  quite  independent  of  whether  these  occur  at 
the  bottom  of  the  sea  or  upon  a  continent  or  island. 

The  recorded  centers  of  disturbed  areas — the  so-called 
"epicenters"  or  "origins" — assembled  for  a  period  of  ten 
years  (1899-1910)  by  Professor  Milne  are  entered  as  the 
spots  upon  the  map  of  Fig.  35.  While  furnishing  a  strik- 
ing confirmation  of  the  accuracy  of  the  map  derived  from 
observations  made  on  the  ground  within  the  earthquake 
districts  of  the  continents,  this  map  tells  us  further  that 
the  continental  earthquake  districts  are  extended  outward 
for  some  distance  upon  the  floor  of  the  neighboring  ocean, 
and  that  the  measure  of  the  disturbances  is  greater  for  the 


THE   PRESENT   REGIONS   OF   RAPID   CHANGE 


97 


undersea  portion  of  the  zone  and  for  the  belts  of  festooned 
archipelagos  southeast  of  the  Eurasian  continent. 

If  now  we  examine  a  modern  atlas  upon  which  the  sea. 
depths  are  indicated,  it  is  at  once  discovered  that  the  dis- 
turbed near-shore  zones  of  the  sea-bottom  are  characterized 
by  generally  narrow  trough-like  depressions  with  steep  walls 
and  with  the  shoreward  wall  generally  continuous  with  the 
precipitous  rising  shore  of  the  neighboring  continent  or 
island  (Fig.  36).  A  section  across  trough-deep  and  rising- 


LEGEND 

.*.-*•  Epicenters  of  Earthquakes  !833-l9IO(Mi)ne)    ^IlGeoSYnclifies  of  Mesozoic  era. 
"—A : — 1~  boundaries  of  areas  which  have  sunk  since  late  Mesozoic  time. 


FIG.  35. — Map  on  Mollweide  projection  to  show  the  distribution  of  world 
earthquakes  for  both  the  land  and  water  areas  of  the  globe  (after 
Seismol.  Comm.  Brit.  Assoc.  Adv.  Sci.) 

shore  together,  and  extended  to  include  the  rising  moun- 
tain range  which  borders  the  shore,  will  generally  be  similar 
to  one  of  those  represented  in  Fig.  36. 

It  will  hardly  escape  the  reader  who  has  followed  the 
discussion  through  the  preceding  chapters,  that  the  nature 
of  the  adjustments  which  are  represented  in  these  sections 
is  exactly  that  developed  in  folding,  and  that  the  elongated 
trough  or  "fore-deep"  represents  a  syncline,  while  the  rising 
mountain  range  presents  the  associated  anticline.  That 
the  development  of  the  anticline  is  responsible  for  the 
formation  of  a  pocket  of  magma  beneath  a  competent 


98        EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

stratum  at  some  proper  depth  is  attested  by  the  fringe  of 
active  volcanoes  which  almost  invariably  follows  the  back 
of  the  anticlinal  ridges  throughout  the  extent  of  these  zones. 
For  a  portion  of  the  belt  these  coordinated  relationships 
are  brought  out  in  Fig.  37,  but  a  good  modern  atlas,  such, 
for  example,  as  Andree's,  will  supply  confirmation  for  the 
zones  throughout. 

GENERALIZED  SECTION  OF  RI3ING  ARC 
(Staff) 


1.  Backland 

2.  Anticline 

&  Synclinal  fore  deep 
1.  Foreland 


INDIVIDUAL  SECTIONS 
(Staff) 


I  Bering  Sea-  Aleutian  Arc-  Aleutian  Fbredeep- Fbcif  ic 
Z.  Japan  Sea- Japan  Arc- Japan  Fbnedeep-Ffacific 
3  East  China  Sea-RiuKiuArc-RiukiuForedaep^  Fbcif  ic, . 
1  Java  Sea  -Java  -Java  Foredeeps-  Indian  Ocean 
5.  Thibet-Himalaya  Arc-Ganges  roredeep-Hindustan 
£  T.tiMcaPlatenu-WeslwnCordillera-AtacantiQfbredeep-Ricinc 

FIG.  36. — Diagrams  to  show  the  form  of  the  sections  of  rising  arcs  (after 

Staff) 

A  study  of  the  distribution  of  sedimentary  rocks  with 
reference  to  their  age,  tells  us  further  that  the  belts  of  ex- 
ceptional disturbance  of  the  earth — of  the  present-day  zones 
of  folding— are  situated  just  where  there  were  narrow  seas 
throughout  the  long  preceding  era  of  the  Mesozoic,  and 
where  in  consequence  sedimentation  was  practically  con- 
tinuous throughout.  Thus  we  see  that  the  seas  of  deposi- 
tion, or  the  geosynclines,  of  one  era  become  the  great  moun- 
tain systems  of  the  next,  and  that  these  in  turn  are  worn 
down  by  erosional  processes  to  supply  the  sediments  for  a 


THE   PRESENT   REGIONS   OF  RAPID   CHANGE 


99 


later  geosyncline.  Thus  is  found  a  doubtless  unintentional 
meaning  for  the  scriptural  language,  "Every  valley  shall 
be  exalted  and  every  mountain  and  hill  shall  be  brought 
low." 

It  will  now  be  in  line  with  the  development  of  our  sub- 
ject, if  we  examine  the  shores  which  are  being  elevated  into 
anticlines  opposite  the  synclinal  fore-deeps  of  the  neigh- 
boring sea.  To  do  this  intelligently  we  must  first  take  ac- 


I_E.6E:ND 

~  Anticlinal  Arcs 
*z>   Pore -deeps 
Active  Volcanoes 


FIG.  37. — The  arcs  of  eastern  Asia  with  their  fore-deeps  ,and  their  fringes 

of  volcanoes 

count  of  the  way  in  which  the  shores  are  being  shaped  by 
the  action  of  the  sea  itself.  The  sea  exerts  its  influence 
directly  upon  the  land  areas  adjacent  chiefly  at  the  times 
of  the  great  storms;  and  so  much  greater  is  the  measure  of 
its  attack  at  the  time  of  an  exceptional  tempest  that  by 
comparison  all  others  pale  into  insignificance.  At  such 
times  its  energy  is  expended  in  drilling,  battering,  and  pry- 
ing away  the  rock  from  any  steeply  rising  shore  as  far 
upward  from  its  quiet  level  as  it  is  able  to  throw  its  mass. 
Undercut  at  the  base,  the  overhanging  rock  falls  in  blocks 


100      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

which  are  themselves  generally  moved  about  and  eventually 
broken  up  to  form  the  sand  of  the  neighboring  beaches  in 
the  lee  of  the  exposed  headlands,  or  else  it  is  transported  by 
the  powerful  undertow  to  build  seaward  the  gently  sloping 
floor  which  is  steadily  invading  the  continent  under  the 
relentless  attack  of  the  waves. 

Though  the  size  and  corresponding  power  of  waves  de- 
pend both  upon  the  intensity  of  the  storm  and,  up  to  a 
certain  limit,  upon  the  expanse  of  sea  over  which  the  waves 
advance  toward  the  shore,  yet  so  broad  are  the  larger  oceans 
with  respect  to  the  diameter  of  storm  areas,  that  it  is  doubt- 
ful if  either  shore  of  the  continent  has  very  greatly  the 
advantage  of  the  other  in  this  respect.  We  may  therefore 
consider  that  the  rate  of  formation  of  a  planed  off-shore 
surface  of  marine  erosion  is  about  the  same  for  the  Atlantic 
as  for  the  Pacific. 

Such  surfaces  of  marine  degradation  surrounding  the  con- 
tinents are  referred  to  as  the  continental  shelves,  and  with  a 
stationary  position  of  the  shore  as  regards  uplift  they 
should  be  of  their  greatest  width.  The  depth  of  the  outer 
margin  of  a  marine  platform  of  degradation  closely  approxi- 
mates to  200  metres  (100  fathoms),  which  is  about  the 
length  of  the  greatest  storm  waves  of  the  ocean  except 
within  the  southern  seas.  This  wave  length  fixes  the  depth 
of  the  outer  margin  of  the  shelf,  and  it  is  therefore  not 
surprising  that  on  the  borders  of  the  Antarctic  continent 
the  waves  of  the  southern  seas  have  graded  the  continental 
shelf  to  a  considerably  greater  depth. 

When  we  now  compare  the  opposite  coasts  of  the  Ameri- 
can continent,  we  find  that  they  are  strikingly  different. 
On  the  map  of  Fig.  38  the  submarine  contours  for  the 
special  depths  of  200  meters  and  2000  meters  are  repre- 
sented, and  together  they  show  that  the  continental  shelves 
which  border  the  Atlantic  coast  of  the  continent  are  rela- 
tively wide,  averaging  not  far  from  one  hundred  miles  in 
breadth,  whereas  those  of  the  Pacific  coast  are  by  contrast 


THE   PRESENT   REGIONS   OF  RAPID   CHANGE 


101 


extremely  narrow.  We  may  interpret  the  Atlantic  shelves 
to  indicate  a  relatively  stationary  position  of  the  continent 
near  this  coast,  thus  confirming  the  observations  assembled 


Contrast  of  "live"  Pacific 
coast  to'cteacTAtlantic 
coast.  Width  of  terrace 
affords  a  measure  of 
the  rate  of  uplift. 

Doffed  1  i  nes  ore  at  depths 
of  SOO  and  of  SOOO  meters 


FIG.  38. — Map  of  the  western  hemisphere  to  show  the  continental  shelves 
(from  Andree's  Handatlas) 

for  earthquakes  and  set  forth  in  Figs.  34  and  35.  The  coasts 
about  the  Arctic  ocean  are  bordered  by  broad  shelves,  and 
are  likewise  to  be  interpreted  as  indicating  that  the  con- 
tinental areas  where  they  border  this  ocean  are  also  rela- 


102      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


The  Continental  5helf  about  the  Arctic  Ocean 

(Nansen) 

Era.  39 


THE   PRESENT   REGIONS   OF  RAPID   CHANGE         103 

lively  immobile  (Fig.  39),  a  fact  confirmed  by  the  earth- 
quake map. 

Yet  the  sea  waves  have  probably  been  doing  their  work 
quite  as  effectively  on  the  Pacific  coast  as  upon  the  Atlan- 
tic. What  then  has  become  of  the  surface  planed  by  wave 
erosion?  The  examination  of  almost  any  section  of  coast 
bordering  the  Pacific  supplies  the  answer  to  this  question. 
Sections  of  this  surface  have  clearly  been  suddenly  lifted 
out  of  the  reach  of  the  waves,  where  they  may  now  be  seen 
in  a  long  succession  in  the  form  of  a  gigantic  staircase  (fig. 
40).  To  make  this  more  impressive,  a  few  examples  out  of 
many  have  been  brought  together  in  fig.  41. 


FIG.  40. — The  form  of  coast  terraces 

Each  of  the  cliffs,  or  risers,  in  the  coastal  staircase  meas- 
ures the  amount  of  the  uplift  of  the  coast  either  during  one 
or  a  succession  of  earthquakes,  although  in  but  few  cases 
can  we  as  yet  affix  the  exact  date  of  this  adjustment.  Ex- 
ceptionally such  terraces  can  be  dated,  as  in  the  case  of 
one  raised  on  the  shore  of  Alaska  during  the  earthquake 
of  1899  (Fig.  42). 

Such  coast  terraces  as  have  been  described  for  the  borders 
of  the  Pacific  Ocean  reveal  the  fact  that  portions  of  the 
sea-floor  are  being  pushed  up  onto  the  continent,  the  testi- 
mony of  the  abrasion  surfaces  which  make  the  treads  of 
the  staircase  being  indubitable.  This  implies  that  a  thrust 
from  the  sea-floor  toward  the  continent  is  continually  oper- 
ating, but  becomes  effective  only  at  intervals  when  the 


104      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

potential  energy  of  the  system  is  let  down  at  the  time  of  an 
earthquake.  This  force  would  be  adequately  explained  if 
the  floor  of  the  sea  within  a  large  area  opposite  the  rising 
shore  were  settling  downward  and  in  consequence  becoming 
reduced  in  area,  as  is  required  by  the  theory  of  secular  cool- 
ing of  the  earth  (Fig.  43). 

If  now  we  inquire  whether  there  is  evidence  that  any 


•ferrate?  of  Coeot Cordillera. fterv(Adooij) 


«o.?ed  Reef  Termc«.Eua.Too9al».(U»«tr) 


Wetan  Ii.(Verbe«K) 
Terraced  Coasts  of  Recent  Uplift  about  the  Pacific  Ocean 


FIG.  41 

large  portion  of  the  Pacific  sea-floor  has  been  settling  since 
the  elevation  of  the  mountains  began  on  its  borders,  an 
answer  is  supplied  for  a  large  area  by  the  study  of  the 
Polynesian  islands  and  their  coral  reefs  and  atolls.  Quite 
independently,  Charles  Darwin  and  James  D.  Dana,  on  the 
basis  of  extended  personal  examination  offered  the  explana- 
tion of  the  low  atolls  within  the  Pacific,  as  a  result  of 
settlement  by  slow  increments  of  former  volcanic  moun- 
tains. These  at  first  emerged  from  the  waves  with  fring- 


THE   PRESENT   REGIONS  OF   RAPID   CHANGE         105 

ing  reefs  about  them,  but  with  progressive  settlement 
became  bordered  by  barrier  reefs  and,  eventually,  when 
the  volcanic  island  had  become  entirely  submerged,  by  the 
upwardly  extended  reef  alone — the  atoll — enclosing  a 
lagoon.  This  view  of  the  origin  of  coral  islands  has  been 
assailed  by  several  biologists,  notably  by  Sir  John  Murray 
and  Alexander  Agassiz,  who  have  offered  a  different  ex- 
planation, but  one  which  has  not  commended  itself  strongly 
to  geologists;  though  an  alternative  theory  has  been 
brought  forward  by  Daly  and  drawn  a  considerable  number 
of  geologists  to  its  support.  The  original  theory  of  Darwin 

OLD  WAVE-CUT  CLIFF 

UPRAISED  5EACM  OF  !892> 
MODERN    BEACH 


FIG.  42.— The  dated  step  in  a  coastal  staircase  in  Alaska  (after  Tarr  and 

Martin) 

has  recently  received  strong  support  in  an  extended  series 
of  studies  by  Professor  W.  M.  Davis,  and  insofar  as  the 
question  of  subsidence  is  concerned  it  is. also  supported  by 
Vaughan.  Outside  the  zone  of  the  tropical  seas  within  which 
corals  can  grow,  we  are  without  direct  evidence  for  sub- 
sidence or  elevation,  but  the  landward  direction  of  the  out- 
ward thrust  from  the  sea-floors  seems  in  itself  to  offer  us  a 
proof  of  a  reduction  of  surface  within  some  neighboring 
region  of  the  ocean  floor. 

But,  it  will  be  said,  the  elevations  of  the  mountains  above 
the  borders  of  the  Pacific  must  just  as  clearly  afford  an  in- 
dication of  extension  of  the  earth's  surface  wherever  the 


106      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

mountains  are  rising,  as  does  the  settlement  of  areas  of 
sea-floor  of  surface  contraction  within  those  areas.  There 
can  be  no  denying  the  fact  that  the  elevation  of  any  belt 
on  the  earth's  surface  with  respect  to  any  relatively  fixed 
datum  like  the  sea  level,  must,  in  and  by  itself,  imply  an 
expansion  of  the  original  surface,  as  indicated  by  the  upper 
diagram  of  Fig.  44.  Such  an  expansion  should  be  displayed 
through  an  extension  of  every  continuous  line  of  metal 
which  crosses  the  district;  such,  for  example,  as  .rails,  pipe 
lines,  etc.  The  effect  will  also  be  shown  in  the  behavior 
of  bridges,  particularly  railroad  bridges,  which  are  parts 
of  a  continuous  metal  line.  Now,  as  has  already  been 
pointed  out  in  an  earlier  chapter,  the  behavior  of  such  struc- 


Diagram  to  illustrate  ttie  relation  of  a  progressive  settlement 
of  tKe  ocean  floor  sector  to  the  elevated  Fbciflc  strands 

FIG.  43 

tures  at  the  tune  of  earthquakes  has  offered  us  the  surpris- 
ing result  that,  instead  of  being  pulled  apart,  they  are 
squeezed  together  and  variously  buckled.  This  can  only  be 
interpreted  to  mean  that  the  area  of  the  sea-floor  which 
has  been  pushed  up  upon  the  continent  and  is  in  full  view 
in  the  coastal  staircase  upon  the  ocean  side  of  the  moun- 
tains, has  been  more  than  sufficient  in  area  to  compensate 
for  the  necessary  expansion  of  the  zone  by  mountain  eleva- 
tion. The  behavior  of  the  rails  and  bridges  indicates  that 
with  each  uplift  to  the  accompaniment  of  earthquakes  the 
thrust  from  the  sea  renews  its  vise-like  compression,  and 
operating  through  a  certain  distance  gains  momentum 
which  is  effective  in  producing  still  greater  compression 
(lower  diagram  of  Fig.  44). 


THE  PRESENT  REGIONS  OF  RAPID   CHANGE         107 

Only  recently  has  it  been  pointed  out  that  on  the  Atlantic 
coast  of  the  United  States  there  exist  elevated  coast  ter- 
races, but  of  a  character  so  different  from  those  of  the 
Pacific,  that  they  have  heretofore  been  overlooked.  The 
reason  for  this  appears  in  part  to  be  that  they  have  been 
so  much  longer  above  the  sea  and  hence  have  been  so  pro- 
foundly changed  by  the  erosive  processes  that  they  can  be 
recognized  only  upon  the  basis  of  precise  measurements 
and  elaborate  synthetical  studies.  Credit  is  due  to  the  late 


MOUNTAIN  GROWTH(ThElORY) 

•  r    5  u  r  fn  _ 


MOUNTAIN  GROWTM(ACTU/AL) 

FIG.  44. — Diagrams  to  explain  the  paradoxical  compression  within  rising 

mountains 

Professor  Barrell  for  having  recognized  this  condition  with- 
in the  New  England  district,  and  his  results,  which  were 
nearly  ready  for  publication  at  the  time  of  his  death,  have 
appeared  in  a  series  of  posthumous  articles.  The  terraces 
which  border  the  Atlantic  coast  are  also  less  easy  to  per- 
ceive for  the  reason  that  the  height  of  the  risers  is  so  small 
in  comparison  with  the  breadth  of  the  treads.  The  con- 
trast which  has  already  been  pointed  out  between  the 
breadth  of  the  eastern  and  western  continental  shelves  of 
the  continent,  is  thus  shown  to  be  continued  upward  in 
the  uplifted  sections  of  the  earlier  shelf;  and  these  latter 


108      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

afford  an  even  better  measure  of  the  comparative  rates  of 
uplift  of  the  coasts,  than  has  already  been  foreshadowed  in 
the  earthquake  maps. 

From  Fig.  45,  in  which  up  to  a  level  of  about  1800  feet 
above  tide  the  Atlantic  terraces  in  New  England  and  the 
Pacific  terraces  of  San  Clemente  Island,  California,  are 
brought  into  comparison;  it  may  be  seen  that  while  the 
Pacific  coast  was  being  elevated  to  this  height  of  about  a 
third  of  a  mile,  the  waves  were  able  to  cut  inland  a  distance 
of  only  about  three  miles,  whereas  on  the  Atlantic  coast 
the  corresponding  elevation  of  the  coast  was  accomplished 
only  after  the  waves  had  cut  back  the  shore  a  distance  of 
some  fifty  miles. 


FIG.  45. — Comparison  of  raised  terraces,  Pacific  and  Atlantic   (from  data 
by  Tangier-Smith  and  Barrell) 

To  extend  the  comparison  so  as  to  include  the  entire 
uplift  upon  the  Pacific  coast,  it  might  be  necessary  to  em- 
ploy the  painstaking  synthetical  methods  of  Barrell  within 
the  higher  levels  in  which  erosion  has  in  part  effaced  the 
characteristic  terrace  contours.  However,  our  comparison 
of  the  two  sets  of  terraces  has  sufficed  to  confirm  the  clear 
indication  of  the  earthquake  maps  that  the  zones  of  the 
earth  which  are  now  undergoing  greatest  change  of  position 
— where  the  surface  shell  of  the  lithosphere  is  relatively 
active — is  the  great  border  zone  of  the  Pacific  and  a  second 
great-circle  zone  which  traverses  the  Mediterranean  Sea  of 
both  America  and  Europe  and  is  continued  eastward  along 
the  course  of  the  former  Sea  of  Tethys — the  twin  plane  of 
the  earlier  lithosphere.  The  maximum  of  surface  mobility 


THE   PRESENT   REGIONS   OF  RAPID   CHANGE         109 

of  our  planet  is  to  be  found  at  the  intersection  of  these  two 
zones  in  the  Moluccas  of  the  Dutch  East  Indies  (see  Fig.  35) . 
It  would  thus  appear  that  when  Sir  Charles  Lyell  suc- 
ceeded in  overthrowing  the  then  accepted  doctrine  of 
catastrophism — calling  for  sudden  and  rapid  geological 
changes — and  supplanting  it  in  the  science  by  his  own  doc- 
trine of  uniformitarianism — supposed  to  demand  almost  in- 
credibly slow  and  gradual  geological  changes — he  failed  to 
appreciate  what  vast  differences  in  these  very  respects  char- 
acterize different  portions  of  the  earth's  surface.  Lyell's 
famous  slogan,  "The  present  is  the  key  to  the  past"  has 
come  to  mean  "The  present  in  Great  Britain,  an  area  of 
extremely  slow  change,  is  the  key  to  the  past."  The  present 
rate  of  change  in  the  region  of  the  southern  Andes  or  in 
New  Zealand,  but  most  of  all  in  the  archipelago  of  the 
Moluccas,  has  a  very  different  meaning.  Though  Lyell 
had  been  a  great  traveller  and  had  brought  to  public  at-' 
tention  the  facts  concerning  a  great  earthquake  in  New 
Zealand,  yet  he  was  not  impressed  by  it  sufficiently  to 
modify  materially  the  form  of  his  doctrine,  modelled  as  it 
was  upon  conditions  in  Great  Britain. 

LITERATURE 

CHARLES   DARWIN.    The  structure  and  distribution  of  coral  reefs,   1874, 

p.  278. 

JAMES  D.  DANA.    Corals  and  coral  islands,  1872,  pp.  398. 
R.  S.  TARR  and  L.   MARTIN.    Recent  changes  of  level  in  Yakutat  Bay 

region,  Alaska,  Bull.  Geol.  Soc.  Am.,  vol.  17,  1906,  pp.  29-64,  pis.  10-23. 
F.  DE  MONTESSUS   DE  BALLORE.    Les  tremblements  de  terre,  Geographic 

Seismologique,  Paris,  1906,  p.  475,  maps  1-2. 
HANS    v.    STAFF.    Zum    Problem    der    Entstehung    der    Umrissform    von 

Celebes,  Zeitsch.  d.  deutsch.  Geol.  Ges.,  vol.  63,  1911,  pp.  180-86. 
H.  H.  TURNER  and  others,  20th  Rept.  of  the  Seismological  Committee, 

Rept.  Brit.  Assoc.  Adv.  Sci.,   Manchester   meeting    (1915),   1916,  pp. 

52-79,  pi.  1. 
W.  M.  DAVIS.    Subsidence  of  reef-encircled  islands,  Bull.  Geol.  Soc.  Am., 

vol.  29,  1918,  pp.  489-574. 
JOSEPH  BARRELL.    The  Piedmont  terraces  of  the  northern  Appalachians, 

Am.  Jour.  Sci.  (4),  vol.  49,  1920,  pp.  227-258,  327-362,  407-428,  pis.  5-6. 
W.  S.  T.  SMITH.    A  geological  sketch  of  San  Clemente  Island,  18th  Ann. 

Rept.  U.  S.  Geol.  Surv.,  pt.  II,  1898,  pp.  459-496,  pis.  84-96. 


110      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

E.  RUDOLPH.     Ueber  die  geographische  Verteilung  der  Epizentralgebiete 

von  Weltbeben  und  ihre  Beziehung  zum  Baue  der  Erdrinde,  C.  R. 

llth  Sess.  Congr.  Geol.  Intern.,  Stockholm,  1910,  1912,  pp.  837-848, 

map. 
A.  AGASSIZ.    The  coral  reefs  of  the  tropical  Pacific,  Mem.  Mus.  Comp. 

Zool.,  vol.  28,  1903. 
R.  A.  DALY.    The  glacial-control  theory  of  coral  reefs,  Proc.  Am.  Acad., 

vol.  51,  1915,  pp.  155-251. 
T.  W.  VAUGHAN.    Fossil  corals  from  Central  America,  Cuba,  and  Porto 

Rico:    with   an  account   of  the   American   Tertiary,  Pleistocene   and 

Recent  Coral  Reefs,  Bull.  103,  U.  S.  Natl.  Mus.,  1919,  pp.  238-332. 


CHAPTER  IX 

THE  CONTRASTED  ASPECTS  OF  THE  EARTH'S  FACE 

IN  an  early  period  of  geological  science  before  accurate 
maps  had  been  prepared  and  before  observational  methods 
of  study  had  been  generally  introduced,  it  was  natural  that 
the  theories  set  up  should  have  borne  but  small  relation 
to  the  facts  they  were  supposed  to  interpret.  Thus  in  the 
first  half  of  the  nineteenth  century  Leopold  von  Buch  in 
Germany  and  filie  de  Beaumont  in  France  attempted  to 
fix  the  age  of  mountain  systems  upon  the  basis  of  their 
directions  or  trends.  Since  it  was  no  easy  matter  in  the 
absence  of  observations  to  disprove  the  claims  which  they 
set  up,  and  as  both  were  men  of  great  intellectual  power 
and  aggressive  in  leadership,  their  theories,  wholly  unsup- 
ported by  fact  as  they  were,  dominated  the  field  of  thought 
of  their  time.  With  the  passing  from  the  stage  of  these 
dominating  personalities,  their  theories  came  almost  at  once 
into  discredit,  and  it  is  a  direct  result  that  attention  was 
withdrawn  from  the  plan  of  the  earth  and  became  concen- 
trated upon  perpendicular  sections  through  the  earth's  outer 
shell  (Fig.  46). 

Such  sections  were  constructed  largely  upon  the  basis  of 
the  observed  attitudes  of  the  hard  rock  strata  at  the  various 
points  where  these  emerge  from  beneath  the  unconsolidated 
materials  generally  forming  the  surface;  and  being  thus 
connected  up  with  actual  observations,  sections  contained  a 
larger  measure  of  truth  than  had  the  crude  theoretical  maps. 
This  profound  change  in  the  direction  of  effort  toward  in- 
creasing observation,  proved  a  most  salutary  one  for  the 
time,  and  it  was  responsible  for  putting  upon  record  a 

ill 


112      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


THE  CONTRASTED  ASPECTS  OF  THE  EARTH'S  FACE     113 

wealth  of  facts  which  alone  could  make  broad  generaliza- 
tions of  any  value. 

While  geologists  were  thus  assembling  the  facts,  geode- 
sists  and  military  and  topographical  engineers  operating 
under  the  direction  of  national  bureaus,  were  slowly  ex- 
tending the  areas  of  surveyed  territory,  as  they  were 
steadily  improving  the  quality  of  the  maps  which  they 
produced.  It  has  been  only  during  the  last  quarter  of  the 
nineteenth  century  and  subsequently  that  attention  has 
.once  more  been  directed  toward  a  study  of  the  broad  plan 
of  the  earth.  This  has  come  about  very  largely  through 
the  influence  of  Eduard  Suess,  whose  masterful  treatise, 
The  Face  of  the  Earth,  is  comparable  in  importance  with 
Lyell's  Principles. 

Suess  was  the  first  to  realize  that  the  crude  geological 
data  which  alone  had  been  available  a  half  century  before, 
quite  as  much  as  the  undeveloped  methods  of  the  time,  had 
been  responsible  for  the  failures  and  for  the  illusions  of 
von  Buch,  de  Beaumont,  and  their  followers.  Yet  with 
all  his  erudition  and  with  a  gift  for  beautiful  and  forceful 
presentation  which  has  been  seldom  equalled,  Suess  was 
none  the  less  poorly  fitted  to  deal  with  the  problems  of 
mechanics  of  earth  movements  which  constituted  a  vital 
element  in  his  studies.  However,  his  enormous  prestige, 
like  that  of  Laplace  in  another  field,  has  carried  some  of 
his  doubtful  conclusions  to  rather  general  acceptance,  at 
least  hi  Europe,  and  without  rigid  tests  having  always  been 
applied  to  them.  Many  of  his  large  generalizations  are 
destined  to  stand  even  when  others  less  securely  founded 
have  been  given  up. 

Perhaps  the  most  valuable  of  all  the  fundamental  con- 
clusions of  Suess  is  his  division  of  the  earth's  continental 
areas  into  two  sections,  one  characterized  dominantly  by 
folding  structures  which  appear  in  the  plan  of  the  earth  as 
great  sweeping  festoon  series  of  mountain  ranges,  and  the 
other  in  striking  contrast  as  alternating  plateau  and  plain 


114      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

— a  mosaic  of  displaced  earth  blocks.  The  first  of  these  is 
generally  characteristic  of  the  continent  of  Eurasia,  and 
the  second  of  that  of  Africa  (Fig.  47).  Though  somewhat 
less  clearly  contrasted,  these  broad  divisions  were  extended 
to  the  western  hemisphere,  the  outer  series  of  festoons  of 
mountains  on  the  eastern  Asiatic  coast  being  connected  up 
through  the  Aleutian  archipelago  with  the  great  Cordil- 
leran  mountain  system  which  generally  borders  the  Pacific 
coast  of  America. 

Another  generalization  of  Professor  Suess  which  is  of 
the  utmost  importance  relates  to  the  coast  lines  on  the 


FIG.  47. — Map  to  show  the  contrasted  aspects  of  the  earth's  face  within 
the  Eastern  Hemisphere  (after  Suess) 

borders  of  the  two  great  oceans,  the  Pacific  and  the  Atlantic. 
On  both  eastern  and  western  coasts  of  the  former  there  are 
mountain  ranges  markedly  festooned  in  arrangement,  and 
taking  their  trend  parallel  to  the  coast  with  their  convex 
side  always  toward  the  sea.  On  the  borders  of  the  Atlantic, 
however,  few  ranges  of  mountains  are  to  be  found,  and  these 
instead  of  trending  parallel  with  the  coast,  meet  it  at  a 
large  angle  and  are,  in  fact,  abruptly  terminated  by  it. 
Such  Rias  coasts,  in  the  view  of  Suess,  indicate  that  the 
adjoining  oceanic  basin  has  been  dropped  down  along 
coastal  fractures  since  the  mountain  ranges  were  formed, 
and  their  continuations  are  presumably  now  removed  from 


THE  CONTRASTED  ASPECTS  OF  THE  EARTH'S  FACE     115 

our  view  and  beneath  the  waters  of  the  ocean.  Professor 
Suess  has  given  strong  grounds  for  believing  that  the  con- 
tinuation of  the  Appalachian  mountain  system  at  the  close 
of  the  long  Paleozoic  era,  instead  of  being  abruptly  termi- 
nated at  the  east  coast  of  Newfoundland,  as  it  is  today,  took 
its  course  eastward  across  a  now  foundered  portion  of  the 
continent  to  be  continued  in  the  eroded  mountain  system 
of  identical  age  which  under  the  names  of  Armorican  and 
Variscan  arcs  may  today  be  found  in  detached  fragments 
stretching  across  western  and  central  Europe  (Fig.  48). 


0 


FIG.  48. — Map  to  show  the  relation  of  the  Permian  mountains  of  Europe 
to  those  of  America 


The  conclusion  from  all  this  has  been,  as  already  pointed 
out,  that  the  Atlantic  basin  was  formed  by  an  inbreak  dur- 
ing Mesozoic  time  of  the  central  portion  of  the  former  con- 
tinent of  which  eastern  North  America  and  western  Europe 
are  the  remnants. 

Within  the  area  south  of  Angara  land  in  eastern  Siberia 
and  extending  eastward  from  the  continent  into  the  festoons 
of  islands  along  the  coast,  as  well  as  southeastward  through 
Malaysia  into  Oceania  so  as  to  mantle  toward  the  east  and 
south  the  coign  of  Australia,  we  encounter  a  series  of  great 
mountain  welts  which  are  primarily  the  result  of  an  intense 
folding  of  strata.  Faulting  has  also  occurred,  but  it  has 


116      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION, 

apparently  been  to  a  large  extent  incidental  to  the  folding 
process.  These  Asiatic  mountain  welts  are  extended  west- 
ward into  southern  Europe  and  extreme  northwestern 
Africa  as  the  ranges  of  the  Caucasus,  Carpathians,  Alps, 
Apennines  and  Atlas  ranges,  together  with  the  less  definite 
ranges  in  the  Balkan  and  Italian  peninsulas.  Looking  east- 
ward from  Asia  we  find  that  the  arcs  of  folded  mountains 
trail  off  northward  along  the  borders  of  the  Pacific,  first 
through  the  Aleutian  arc  and  thence  down  through  the 
long  Cordilleran  backbone  of  the  western  hemisphere  to 
join  up  with  an  arcuate  hook  which  projects  out  from  the 
Antarctic  continent  to  form  Palmer  Land  or  West  Antarc- 
tica. 

In  northern  Europe  between  the  Baltic  shield  and  the 
southern  European  arcs  extending  northwestward  to  the 
Atlantic,  as  in  North  America  between  the  Laurentian 
coign  and  the  northern  Cordilleran  system  of  the  continent, 
we  find  areas  which  are  much  less  easily  classified.  Within 
them  are  the  eroded  remnants  of  much  earlier  mountain 
arcs  which  date  from  periods  so  much  more  remote  hi  geo- 
logical history  that  their  plan  is  much  less  easily  read.  The 
main  lines,  however,  we  can  still  make  out. 

The  study  of  ancient  geography  has  revealed  clearly  the 
fact  that  though  the  ocean  basins  have  not  generally  been 
permanent,  but  have  in  the  course  of  geological  ages  been 
profoundly  transformed,  the  continental  shelves  with  their 
shelf  seas  over  which  marine  sediments  have  been  laid 
down,  have  persisted  throughout  long  ages  as  continuous 
floors  of  deposition,  and  that  they  have  subsequently  been 
elevated  to  form  the  great  folded  mountain  systems  of  the 
earth.  Such  long  marginal  bands  of  marine  sediments  sur- 
rounding the  earlier  continental  areas  have  been  denomi- 
nated geosynclines,  and  the  best  known  of  them  belong  to 
the  early  and  the  middle  eras  of  earth  history,  the  Paleozoic 
and  the  Mesozoic  eras  (see  Fig.  35). 

The  Paleozoic  geosynclines  were  elevated  into  mountains 


THE  CONTRASTED  ASPECTS  OF  THE  EARTH'S  FACE     117 

at  the  close  of  that  era  and  are  best  known  to  us  in  the 
Appalachian  ranges  composed  of  folded  beds  which  involve, 
as  usually  assumed,  from  30,000  to  50,000  feet  of  sediments. 
The  great  geosynclines  of  the  next  geological  era  were  zones 
of  continuous  sedimentation  throughout  Mesozoic  time,  at 
the  conclusion  of  which  they  were  similarly  raised  to  form 
what  are  now  the  greatest  mountains  of  the  earth.  They 
include  the  Himalayas  with  their  associated  ranges  and  ex- 
tensions to  the  east  and  west  across  Eurasia,  and  the  great 
Cordilleran  system  of  the  western  continent.  Thus  the 
series  of  Mesozoic  geosynclines  marked  out  the  course  of 
the  present  great  mountain  systems,  and  they  have  under- 
gone less  subsequent  change  than  have  those  mountains 
which  were  reared  above  the  Paleozoic  geosynclines,  and 
their  courses  like  their  structures  are  so  much  the  more 
easily  followed. 

Confirmation  of  the  view  that  the  outermost  and  most 
recent  welts  of  the  Mesozoic  geosynclines  are  still  in  process 
of  erection,  is  supplied  by  the  line  of  active  volcanic  vents 
which  parallels  the  series  and  almost  completely  surrounds 
the  ocean  as  the  so-called  "fire  ring"  or  "fire  girdle"  of  the 
Pacific  (see  frontispiece).  It  is  especially  noteworthy  in 
view  of  what  has  been  said  concerning  the  magma  cham- 
bers (see  Chapter  III),  that  these  volcanoes  generally  lie 
upon  the  inner,  or  back,  side  of  the  arcs  away  from  the  ocean. 
Upon  the  westward  border  of  the  Pacific  the  outermost 
series  of  welts,  instead  of  resting  upon  the  continent,  rises 
from  the  depths  of  the  sea,  sometimes  with  other  only  less 
recent  island  arcs  at  its  back,  between  it  and  the  continent. 
In  front  of  the  outer  series  of  island  arcs,  the  sea  bottom 
generally  drops  down  into  long  and  narrow  troughs — fore- 
troughs — running  parallel  to  the  arcs.  Behind  the  island 
arcs  are  usually  other  deeps  less  profound  and  lacking  the 
characteristically  long  and  narrow  trench  form,  and  these 
we  may  call  "back-deeps"  rather  than  troughs  (Fig.  37). 
Less  strikingly  trench-like  also,  somewhat  similar  deeps  lie 


118      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

either  close  in  near  the  Pacific  coast  of  North  America  or 
else  as  an  intermontane  valley  upon  the  continent  between 
a  coast  range  and  a  cordillera  at  its  back.  Excepting  only 
those  Atlantic  arcs  which  lie  within  the  zone  of  the  former 
twin  plane  extended  in  the  Sea  of  Tethys,  nothing  at  all 
resembling  these  fore-deeps  on  the  Pacific  is  to  be  found 
near  the  Atlantic  coast  or,  in  fact,  within  the  Atlantic  basin. 

LITERATURE 

EDUARD  SUESS.  Die  Entstehung  der  Alpen,  1875. 
EDUARD  SUESS.  The  face  of  the  earth,  1885-1909. 
W.  M.  DAVIS.  The  framework  of  the  earth  (review  of  the  French  edition 

of  "The  Face  of  the  Earth"),  Am.  Jour.  Sci.  (4),  1919,  pp.  195-241. 
W.  H.  HOBBS.    Mechanics  of  formation  of  arcuate  mountains,  Jour.  Geol., 

vol.  32,  1914,  part  1,  pp.  71-79. 
SMILE  HAUG.    Les  geosynclinaux  et  les  aires  continentales,  Contribution  a. 

1 'etude  des  transgressions  et  des  regressions  marines,  Bull.  de.  la  Soc. 

Geol.  de  France  (3)  vol.  28,  1900,  pp.  617-711,  especially  fig.  1,  opp. 

p.  632. 
THEODOR  ARLDT.    Entwicklung  der  Kontinent  und  ihrer  Lebewelt,  Leipzig, 

1907,  especially  plate  labelled  "Gebirgskarte"  at  end  of  volume. 
FERDINAND   VON    RICHTOFEN.    Geomorphologische    Studien    aus    Ostasien, 

Sitzungsber.  d.  k.  preusz.  Akad.  d.  Wiss.,  1900,  pp.  888-925;  ibid.,  1901, 

pp.  782-808;  ibid.,  1902,  pp.  944-975. 
FEDERICO  SACCO.     Les  lois  fondamentales  de  1'orogine  de  la  terre,  Turin, 

1906,  p.  26,  plate. 
FRANK  B.  TAYLOR.    Bearing  of  the  Tertiary  mountain  belt  on  the  origin 

of  the  earth's  plan,  Bull.  Geol.  Soc.  Am.,  vol.  21,  1910,  pp.  179-226. 
E.  C.  ABENDANON.    Die   Grpssfalten  der  Erdrinde,  Leiden,   1914,  p.   183. 
K.  ANDREE.    Ueber  die  Bedingungen  der  Gebirgsbildung,  Berlin,  1914. 

V 


CHAPTER  X 

THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE 

IT  has  been  pointed  out  in  the  last  chapter  that  upon 
the  Eurasian  continent  generally,  as  well  as  throughout  the 
zone  which  extends  around  the  Pacific  Ocean,  the  earth's 
face  is  traversed  by  a  wealth  of  curving  creases,  whereas 
the  continent  of  Africa  presents  a  wholly  different  aspect, 
In  the  former  region  the  surface  expression  of  the  relief  is  a 
result  primarily  of  the  process  of  folding,  with  faulting 
playing  a  generally  secondary  role;  whereas  in  the  latter  the 
surface  configuration  is  largely  a  direct  expression  of  the 
faulting  process,  and  the  result  a  pattern  on  first  sight  re- 
sembling perhaps  a  Chinese  puzzle,  but  on  closer  examina- 
tion the  more  regular  design  of  marquetry. 

For  the  present  it  is  the  pattern  of  the  mountain  wrinkles 
within  the  folded  regions  which  will  engage  our  attention, 
and  it  is  necessary  first  of  all  to  rid  ourselves  of  certain 
preconceived  notions  derived  from  our  study  of  geography 
in  the  schools.  It  was  long  the  custom  to  represent  moun- 
tain ranges  upon  small-scale  maps  where  the  character 
rather  than  the  actual  positions  of  the  ranges  is  to  be  pre- 
sented, as  though  they  took  their  courses  in  a  sinuous  or 
serpentine  fashion.  Most  persons  who  have  now  reached 
maturity,  and  it  is  not  intended  to  say  how  many  others, 
think  of  mountain  chains  in  terms  of  the  appearance  which 
these  features  made  upon  the  maps  of  their  school  atlases,  a 
or  b  of  Fig.  49 ;  whereas  for  nearly  a  generation  those  who 
have  given  intimate  attention  to  the  problems  of  mountain 
structure,  know  that  mountain  chains  are  really  nat  sinuous, 
but  scalloped,  as  schematically  represented  in  c  of  Fig.  49. 

Geologists  and   geographers  are  themselves  largely   to 

119 


120      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

blame  for  this  prevalent  misconception,  for  ever  since  the 
passing  from  the  stage  of  von  Buch  and  de  Beaumont  with 
their  weirdly  distorted  views  of  mountain  plans,  they  have 
as  a  profession  and  in  a  spirit  of  revolt  against  the  earlier 
false  doctrines,  confined  their  attention  largely  to  the  study 
of  vertical  sections  through  the  rock  strata,  rather  than  to 
the  arrangement  of  the  mountain  creases  in  the  plan  of 
the  earth. 

To  prepare  sections  the  local  inclinations,  the  dips,  of  the 
strata  are  studied,  whereas  to  decipher  the  plan  of  arrange- 
ment of  folds,  it  is  the  trend  or  bearing  of  the  strata,  their 
strike,  which  must  be  noted  wherever  they  emerge  from 
beneath  their  cover.  It  is  already  apparent  that  a  change 


FIG.  49. — Earlier  (incorrect)  and  (correct)  methods  of  representing  moun- 
tain systems  upon  maps 

in  method  is  coming  in  the  science  and  it  requires  no 
prophet  to  declare  that  the  present  emphasis  upon  dip 
geology  will  in  the  future  in  considerable  measure  be  sup- 
planted by  attention  devoted  to  strike  geology. 

It  will  be  remembered  that  two  of  the  great  generaliza- 
tions of  Suess  concerning  the  plan  of  the  curving  creases  or 
arcs  of  mountains  were,  that  their  convex  side  is  always 
presented  toward  the  existing  oceans,  and  that  they  gen- 
erally trend  parallel  with  the  ocean  borders.  An  additional 
generalization  of  equal  importance  which  Suess  was  the 
first  to  discover,  is  that  within  these  arcs  the  folds  are  un- 
symmetrical  with  their  steeper  sides  toward  the  existing 
oceans.  His  manner  of  expressing  this  has  been  that  the 
regions  "are  folded  in  the  direction  of  the  ocean" ;  a  state- 
ment involving  a  theory  of  arc  formation  which,  it  will 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     121 


later  be  pointed  out,  we  believe,  to  be  unsound,  notwith- 
standing the  fact  that  it  is  generally  accepted  today  by  those 
who  have  given  attention  to  the  subject.  We  have  chosen, 
therefore,  to  put  his  generalization  in  the  form,  the  steeper 
side  of  the  old  faces  the  ocean,  so  as  to  indicate  observed 
fact  quite  independent  of  any  theory  of  origin. 

In  the  last  volume  of  The  Face  of  the  Earth,  which  Pro- 
fessor Suess  published  in  1885,  a  full  score  of  years  after 
the  foregoing  generalization  had  been  first  pronounced  as 
a  folding  of  the  strata  "in  the  direction  of  the  ocean,"  Suess 
felt  compelled  to  modify  the  statement  insofar  as  concerned 


V.; 


Map  of  Java  with  Foredeep3(Brouwer) 
FIG.  50 

the  arcs  of  the  western  hemisphere  to  the  southward  of  the 
49th  parallel  of  north  latitude,  and  to  describe  this  great 
exception  to  his  generalization  in  which  the  folding  was 
"away  from  the  ocean"  instead  of  toward  it,  as  Andinenbau 
or  Andean  structure.  It  will  be  shown  that  if  Suess' s  theory 
of  origin  of  the  arcs  be  discarded  for  the  view  here  set  forth 
that  mountains  have  been  raised  as  a  result  of  thrusts  going 
out  from  areas  of  subsidence,  generally  the  ocean  areas,  in- 
stead of  from  the  continental  regions,  the  exception  of  the 
Andean  structure  has  no  reality,  since  the  American  Cordil- 
lera at  the  tune  of  its  formation  faced  a  sea  upon  its  eastern, 
and  not  its  western,  flank. 


122      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

We  owe  also  to  Professor  Suess  the  important 
generalization  that  in  the  case  of  the  arcs  forming 
the  outer  margin  of  a  series,  there  are  fore-deeps 
along  their  fronts  and  fringes  of  volcanoes  at  their 
backs  (see  Fig.  35).    As  a  case  in  point  we  may 
cite  the  great  sweeping  arc  of  the  Malayan  archi- 
pelago which  includes  Sumatra,  Java  and  the 
lesser  islands  to  the  eastward,  and  which  fronts 
g      the  broad  expanse  of  the  Indian  Ocean  to  the 
g     south  and  west.    In  Fig.  50  is  represented,  mainly 
f£      after  Brouwer,  the  island  of  Java  with  its,  in  this 
1      case,  double  fore-deep,  and  in  Fig.  51  is  shown  a 
^      geological  section  across  the  island  with  the  un- 
£      symmetrical  folds  facing  the  sea  at  the  front  with 

>  their  steeper  limbs. 

js         Suess  was  led  to  his  pregnant  generalizations 
£     concerning  arcuate  mountains  from  a  study  of  the 

>  "Origin  of  the  Alps,"  which  he  brought  out  as 
early  as  1875  and  in  which  the  trend  that  his 

0  views  were  later  to  take  is  already  clearly  fore- 
shadowed.   From  the  outline  map  of  the  Alpine 

3  region  of  Fig.  52  it  appears  that  a  large  area  of 

1  granite  and  associated  igneous  and  metamorphic 
1  rocks  which  has  become  known  as  the  "Bohemian 
j|  Mass,"  lies  opposite  and  almost  within  the  re- 
J  entrant  where  the  strongly  marked  arcs  of  the 

I.      Alps  and  the  Carpathians  are  united.    This  Bo- 
^     hemian  mass  early  took  on  in  the  mind  of  Suess 
£      the  character  of  a  firmly  rooted  and  unyielding 
obstruction,   which   by  halting  and  holding   in 
check  a  generally  northwardly  directed  migration 
of  the  area  to  the  southward,  permitted  those 
portions  toward  the  flanks  to  advance  northward 
and  in  part  enclose  the  obstruction. 

When  now  Suess  had  extended  his  studies  to 
include  the  continent  of  Asia  with  its  even  more 
perfect  examples  of  arcuate  mountain  creases, 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     123 

he  appears  to  have  been  no  less  strongly  impressed  by 
the  supposed  unyielding  quality  and  significant  posi- 
tion of  a  somewhat  similar  mass  of  ancient  granite 
and  gneiss  which  he  called  the  "Indian  Mass,"  and 


j 


FIG.  52. — Sketch  map   of  southern  Europe  to  show  the  position  of  the 
"Bohemian  Mass"  with  reference  to  the  mountain  arcs 

which  was  situated  similarly  opposite  the  most  striking  re- 
entrant of  the  Asiatic  series  of  arcs  (Fig.  53).  These  two 
masses  appear,  however,  to  be  each  unique  within  its  own 
region  and,  like  the  rings  of  Saturn  in  the  astronomical  field, 
they  appear  to  have  been  responsible  for  the  form  which  a 


I    n  d    i   a    n 

Oce  a  n 


FIG.  53. — Sketch  map  of  southern  Asia  to  show  the  position  of  the  "Indian 
Mass"  with  reference  to  the  mountain  arcs 

great  theory  was  to  assume;  for  Suess  became  confirmed  in 
his  view  that  the  portions  of  the  land  areas  lying  within 
the  arcs  moved  forward  over  the  portions  in  front,  and  so 
produced  the  creases  by  a  process  of  overturning  from  the 
rear  toward  the  front  of  the  arc.  The  nature  of  this  move- 


124      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

ment,  as  he  conceived  it,  is  quite  analogous  to  that  which 
takes  place  in  the  surf,  where  the  upper  layer  of  water  in  a 
wave  advancing  toward  the  shore  rides  forward  over  the 
lower  and  so  results  in  an  overturning  of  the  layers  from 
the  back  toward  the  front  (a  and  b,  Fig.  54). 

Under  the  conception  of  Suess  it  is  the  land  areas  of  the 
continents  which  in  their  upper  portions  are  migrating  sea- 
ward and  over-riding  the  layers  below,  which  are  therefore 
conceived  to  be  in  a  state  of  relative  repose — they  offer 
passive  resistance  only.  It  is  our  belief,  as  was  pointed  out 
in  a  series  of  papers  published  in  the  Journal  of  Geology 


FIG.  54. — Diagrams  to  illustrate  the  contrasted  views  of  Suess  and  the 
author  concerning  the  direction  of  the  active  thrust  in  mountain 
evolution,  a,  surf;  b,  Suess  view;  c,  author's  view 

as  long  ago  as  1912,  that  the  migration  of  strata  which  be- 
come involved  in  the  formation  of  arcuate  mountains  is 
exactly  in  the  reverse  direction  from  what  Professor  Suess 
believed  it  to  be  (c,  Fig.  54).  The  mountain  arcs  face  the 
sea,  not  because  the  land  is  thrusting  itself  seaward  from 
within  the  continental  areas  behind  them,  but  because  the 
strata  beneath  these  sinking  sea  bottoms  at  their  front  find 
themselves  ever  more  closely  pinched  because  of  their  set- 
tlement, and  in  consequence  are  exerting  a  thrust  outward 
against  the  continent.* 

Such  a  formation  of  arcuate  creases  by  a  process  of  under- 
turning,  we  demonstrated  experimentally  as  to  its  me- 
chanics by  the  simple  device  shown  in  plate  VI.  This  is  a 

*  So  far  as  the  author  is  aware,  the  only  geologist  who  has  advanced 
the  view  that  the  thrust  in  mountain  building  has  been  exerted  toward 
the  continent  from  the  ocean  is  Willis,  and  he  in  connection  only  with 
his  later  studies  in  China.  The  cause  is  in  his  opinion  the  "undertow" 
resulting  from  isostatic  adjustments.  (Bailey  Willis,  Research  in  China, 
vol.  II,  Systematic  Geology,  Carnegie  Inst.  of  Washington,  1907,  pp. 
123-133. 


PLATE  VI 


Experiment  for  producing  arcuate  wrinkles  by  underturning  of  plastic 
Canada  balsam  during  the  contraction  of  a  rubber  sheet.  1.  The  apparatus. 
2.  The  effect  on  the  balsam. 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     125 

rigid  triangular  frame  enclosing  elastic  adjustable  walls 
which  permit  of  attaching  to  them  an  india  rubber  mem- 
brane which  can  therefore  be  stretched  or  released  at  will. 
On  the  surface  of  the  membrane,  which  is  made  of  the  ma- 
terial in  common  use  by  dentists  for  rubber  dams,  is  poured 
Canada  balsam  after  reducing  it  to  a  consistency  from 
which  it  quickly  cools  and  remains  rigid.  When  this  balsam 
layer  has  been  added  as  a  coating  to  the  membrane  and 
before  it  has  had  time  to  solidify,  a  mass  of  cold  metal  is 
brought  in  contact  with  the  membrane  from  below  and  a 
local  area  cooled  and  congealed.  The  membrane  is  now 
allowed  to  contract  and  the  still  viscous  balsam  surround- 
ing the  rigified  "shield"  allowed  to  crease  by  underthrust 
from  without,  whereupon  it  quickly  congeals  in  definite  arcs 
as  shown  in  plate  VI  at  the  bottom. 

The  essential  contrast  of  the  two  views  of  arc  formation 
is  brought  out  in  Fig.  54,  in  which  b  represents  the  view  of 
Suess  regarding  the  positions  of  the  active  force  and  of 
migrating  strata  with  reference  to  the  passive  forces,  and 
c  that  of  the  author.  In  the  author's  view,  we  would  re- 
peat, it  is  the  layers  of  strata  at  the  front  that  are  forced 
under  those  situated  at  a  higher  level  near  the  coasts  to 
produce  underturned  folds  from  the  front  of  the  arc,  and 
not  overturned  folds  from  its  top  and  rear.  As  we  shall  see 
later,  the  mechanics  of  the  process  of  arcuate  folding  per- 
mits of  local  lateral  overthrust  inward  in  the  case  of  strongly 
curving  arcs,  but  not  overthrust  outward,  as  was  assumed 
by  Suess. 

It  must  further  be  clear  that  upon  the  author's  hypoth- 
esis, it  is  a  vast  region  which  supplies  the  compression 
resulting  hi  a  reduction  of  superficial  area  so  that  the 
"slack"  of  the  strata  is  taken  up  within  the  folds.  In  the 
conception  of  Suess  it  is,  on  the  contrary,  a  distinctly  cir- 
cumscribed area  within  the  arc  which  is  involved,  and  which 
must  therefore  actually  expand  to  produce  the  outward 
migration  of  the  strata  to  form  the  arc  (Fig.  55).  Such 


126      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


an  expansion  of  the  strata  must  certainly  require  a  note- 
worthy thinning  of  the  strata  themselves  within  the  area 
where  they  are  expanding,  and  such  variation  in  thickness 
must  appear  in  the  folds  themselves  as  a  thinning  of  the 
upper  limbs  in  a  manner  which  is  brought  out  in  Fig.  55, 
in  which  the  contrasted  views  are  graphically  expressed. 
The  form  of  fold  required,  which  is  shown  at  the  left  in 
the  figure,  has  seldom,  we  believe,  been  observed;  whereas 
the  other  form  required  by  the  theory  of  underturning  from 
in  front  is  the  normal  type  within  all  areas  of  intensely 
folded  rocks. 


R£gT5fi  of 
More  Rigid    floss 


<«  f  iV* 

'*WA«  ° 

I 

Form  of  Fold 


Upp«r    Limb    Thinned 


Under   Limb  Thinned 

HOBB5J9IZ 

Contrast  of  opposing  views  concerninq  the  origin  ofarcuate  mountains 
FIG.  55 

If  the  active  force  which  induces  folds  and  yields  arcuate 
mountain  creases,  is  one  which  goes  out  from  sinking  areas 
of  the  ocean  floor  it  becomes  possible  also  to  study  the  me- 
chanics of  those  mountain  systems  which  have  been  reared 
hi  the  remote  past  on  the  borders  of  former  oceans,  since 
these  latter  may  now  be  mapped  upon  the  basis  of  the  dis- 
tribution of  their  sediments.  If,  on  the  other  hand,  such 
forces  have  gone  out  from  the  land  areas  we  are  prevented 
from  carrying  out  such  studies  by  the  fact  that  ancient  land 
areas  are  much  more  difficultly  located,  their  materials  hav- 
ing long  since  been  scattered  in  many  directions  and  been 
mingled  in  the  deposits  laid  down  in  their  time. 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     127 

It  seems  to  have  been  overlooked  that  Professor  Suess 
nowhere  in  his  great  treatise  has  suggested  why  the  rock  of 
the  continental  interiors  should  migrate  outward  toward  the 
surrounding  seas.  To  invoke  the  force  of  gravitation  is  ob- 
viously beside  the  point. 

We  may  begin  now  by  considering  the  conditions  which 
marked  the  close  of  the  long  Paleozoic  era  when  the  con- 
tinents stretched  in  east  and  west  directions,  with  a  north- 
ern coast  line  of  Appalachia-Armorica  extending  across  the 
present  Atlantic  from  Newfoundland  to  Brittany  and  facing 
an  ocean  to  the  northward.  The  mountain  arcs  on  this  coast 
line  are  represented  in  fig.  56,  and  ranged  behind  them  we 


/Ancient  Coigns 
Permo-Ccrbon.  Ocean 
->•• — Pen-no  -Carbon.  Arcs 
*,+   Permian  Volcanoes 

Mountain  Arcs  of  the  Northern  Hemisphere 
Formed  at  close  of  the  Poleoro.c   Era- 

FIG.  56 

find  the  present  remains  of  the  igneous  materials  which 
took  part  in  the  mountain  building  process.  For  the  eastern 
United  States,  these  facts  are  set  forth  on  a  larger  scale  in 
the  map  of  Fig.  57,  the  geosyncline  of  the  long  preceding  era 
having  determined  the  position  of  the  belt  within  which 
the  arcs  are  developed.  A  few  examples  of  the  forms  of 
folds  are  also  supplied  to  show  how  they  were  underturned 
from  the  direction  of  the  then  ocean  lying  to  the  westward. 
The  point  of  importance  to  be  noted  is  that  here,  as  appar- 
ently in  all  cases,  the  steeper  limbs  of  the  folds,  and  the 
thrusts  whenever  formed,  face  the  basin  in  front,  by  the 
settlement  within  which  the  folding  is  believed  to  have 
been  determined. 


128     EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


The  fact  has  already  been  alluded  to,  that  Professor  Suess 
felt  compelled  to  modify  his  earlier  generalization  that  arcs 
in  all  cases  face  the  existing  oceans  with  the  steeper  sides 
of  their  folds,  by  later  making  exception  for  that  portion  of 


Pcrmo-Corb.  Arc*. 

>  Direction  of  Thrust. 

•     Feldspathoid  Magmas. 
<fl»   Gabbroitic(Basdtlc)  Magmas. 
0  6ronitic(Rhyolitic)  Maomas. 


Chimney  Top  Section,  Te 


Rome  Section,  Ga. 


FIG.  57. — Map  of  the  arcs  of  the  eastern  United  States  with  geological 

sections 

the  great  Cordillera  of  the  western  hemisphere  which  lies 
to  the  southward  of  British  America.  His  later  reference 
to  his  first  statement  is  that  this  earlier  statement  "was  in 
accordance  with  the  observations  which  had  been  made  up 
to  that  time."  This  later  modification,  printed  in  the  fourth 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     129 

volume  of  his  masterpiece,  was  made  after  Willis,  Daly  and 
others  had  brought  out  their  sections  from  near  the  49th 
Parallel  of  North  Latitude  and  shown  that  great  thrusts 
have  in  the  Rocky  Mountains  taken  place  with  folds  which 
faced  eastward,  or  away  from  the  Pacific,  instead  of  west- 
ward as  had  at  first  been  supposed.  For  the  great  Andean 
system  of  South  America  the  fact  that  the  steeper  limbs  of 
the  folds  face  away  from  the  Pacific  had  been  brought  out 
by  Steinmann  (Fig.  20,  p.  58),  and  to  this  evidence  we  may 
now  add  a  section  by  Palmer  (Fig.  58).  Today  we  may  ex- 
tend the  exceptions  cited  by  Suess  in  the  other  direction  so 
as  to  carry  them  northward  to  the  Arctic  Ocean  and  state 
that  for  the  entire  Cordilleran  backbone  of  the  western  con- 

EA57IRN  CORDILLERA  PRE' CORDILLERA  THE- PAMPAS 


Paleozoic. 
Quortzites 
Limestone          I  Red  Sandstone 


Section  at  east  baae  of  Andes, N.W.Arqentina(Ralmer) 

FIG.  58. — Geological  section  across  the  eastern  base  of  the  Andes  in  north- 
western Argentina  (after  Palmer). 

tinent  the  folding  is  away  from  the  Pacific  in  the  sense  of 
Suess;  i.e.,  the  steeper  limbs  of  the  folds  are  on  the  eastern 
rather  than  on  the  western  sides.  In  order  to  make  this 
clear  for  the  areas  to  the  northward  of  Colorado,  the  sec- 
tions of  Fig.  59  have  been  assembled. 

The  difficulties  encountered  by  Suess  in  bringing  the 
American  Cordillera  within  his  scheme,  was  wholly  due  to 
the  fact  that  he  was  requiring  the  continents  to  move  out- 
ward toward  the  oceans  by  overthrust.  When  now  we  look 
upon  the  fold  arcs  as  raised  by  thrusts  from  the  seas  on 
whose  borders  they  are  being  elevated,  the  American  Cor- 
dillera conforms  absolutely  to  the  law  which  elsewhere 
prevails. 

As  is  well  known,  the  Rocky  Mountain  system  began  to 
be  elevated  in  Laramie  (late  Cretaceous)  time  when  a  de- 
pression was  formed  to  the  eastward,  as  can  be  seen  by  in- 


130      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

spection  of  the  map  of  Fig.  60.  The  Coast  Ranges,  which 
face  the  Pacific  directly,  betray  no  indication  of  such  a  di- 
rection of  movement,  and  the  sections  thus  far  prepared  in- 


Along  151*  Mend.,  Alaska  (Schroder) 


So. Fork  of  6ho«t  Riv.  B.C.  ( MS  Connel  I) 


Cascade  Coal  Basin  (Dowling) 


Moose  Mt.  Region  ,B.C.  (Cairns) 


Along 49T-*Paral lei  (Daly) 


Lewis  ana;  Livingston  Ranges,  Mont. (Wil  lis) 


Along  43BPPc1ralle,l  (Richards  and  Mansfield) 

Geological  Sections  of  the  Rocky  Mountains 
facing  the  Laramide  Sea 

FIG.  59 

dicate  for  the  direction  of  thrust  the  exactly  opposite  quar- 
ter, insofar  at  least  as  the  late  Mesozoic  and  Cenozoic  for- 
mations are  concerned.  The  trend  of  the  formations  also  in- 
dicates an  arcuate  structure  convex  toward  the  Pacific  and 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     131 


with  vertices  or  reentrants  at  Puget  Sound  and  also  near  the 
international  boundary  with  Mexico  (Fig.  61). 

Subsequent  to  the  formation  of  the  series  of  arcs  forming 
the  eastern  and  western  boundaries  of  the  broad  area  sepa- 


•* —  Direction  of  thrust 
when  known  from  sections. 


FIG.  60. — Map  of  the  Cretaceous  Ocean  and  the  Rocky  Mts.  which  rose  on 

its  border 

rating  the  Rocky  Mountain  Ranges  from  the  Pacific  coast, 
there  came  the  settlement  of  a  vast  interior  area  now  repre- 
sented by  the  "Great  Basin"  and  its  extensions  to  the  north- 
ward. This  settlement  had  the  effect  of  introducing  new 


132      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

thrusts  which  were  directed  westward  on  the  west  side 
toward  the  present  Sierra  Nevadas,  and  eastward  on  the 
east  side  toward  the  Bitter  Root,  Wasatch,  and  other  ranges 
which  there  form  the  boundary  of  the  basin  (Fig.  62).  The 
immense  development  of  contemporaneous  or  later  igneous 
rocks  has  been  largely  responsible  for  the  small  number  of 


UNITED  STATES 


Neocene 
HZ3        Eocene 
•••        Jura-Triaa 
EH)        Met.  Paleozoic 

fault  of  l906(Gilber» 
------    lOOrathom  Line 

FIG.  61 


good  sections  in  which  the  structures  of  the  sedimentary 
rocks  can  be  clearly  made  out,  but  we  none  the  less  have 
one  excellent  section  from  the  western  border,  Diller's  Tay- 
lorville  section  in  California,  and  also  sections  through  the 
Wasatch  Range  on  the  eastern  border. 

The  contrast  between  the  Suess  conception  as  given  in 
his  later  modified  statement,  and  the  view  here  set  forth  by 
the  author,  is  schematically  represented  in  Fig.  63. 


THE  MIGRATIONAL  MOVEMENTS  OF  THE  SURFACE     133 


!< 


X 


f -Hnr\ 


\v 


if-; 


FIG.  62. — Sketch-map  of  the  great  depression  formed  in  Pleistocene  time, 
between  the  Rocky  Mts.  and  the  Pacific  Coast  of  the  United  States, 
together  with  sections 


134      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


Q& 


5uess  Conception  (  Later) 


Coast 

Mta 


Rocky 
Mb. 


Cretaceous-Tertiary  Deformation 


Coast-     Sierra 
Mts       Nevada 

Mt3., 


Wasateh    Rocky 
Mts.        Mta. 


\ 


Quaternary  Deformation 
Contrasted  views  of  Suess  and  Hobbs  concerning  origin 
.of  the  mountain  arcs  of  the   western  United  States 

FIG.  63 


LITERATURE 

K.  VON   ZITTEL.     History   of  Geology   and  Paleontology    (translation  by 

Ogilvie-Gordon) ,  London,  1901,  pp.  61,  64,  299. 
EDUARD  SUESS.    Die  Entstehung  der  Alpen,  Vienna,  1875. 
EDUARD  SUESS.    The  face  of  the  earth,  5  vols. 
EDUARD   SUESS.    On   the   asymmetry   of   the   northern  hemisphere,   Scot. 

Geogr.  Mag.,  vol.  14,  1898,  pp.  649-654. 
WILLIAM  H.  HOBBS.    Mechanics  of  formation  of  arcuate  mountains,  Jour. 

Geol.,  vol.  22,  1914,  pp.  71-90,  166-188,  193-208. 
BAILEY     WILLIS.    Stratigraphy    and    structure,    Lewis    and    Livingstone 

Ranges,  Montana,  Bull.  Geol.  Soc.  Am.,  vol.  13,  1902,  pp.  305-352, 

pis.  46-53. 
BAILEY  WILLIS.    Research  in  China,  vol.  2,  Systematic  geology,  Carnegie 

Institute  of  Washington,  1907,  pp.  433,  pi.  10. 


CHAPTER  XI 

THE   PATTERNS   OF   THE   FACIAL  WRINKLES   AND 
THEIR  MEANING 

HAVING  now  shown  that  the  rule  of  direction  of  folding 
within  arcs  on  the  earth's  face  is  not  subject  to  an  important 
exception  for  the  western  hemisphere,  as  Suess  had  sup- 
posed, we  are  in  a  position  to  view  the  general  plan  of  the 
wrinkles  which  cross  the  present  earth's  face  (see  frontis- 
piece) and  to  inquire  a  little  further  into  the  law  of  their 
formation. 

Our  discussions  of  the  folds  within  the  arcs  have  thus  far 
been  centered  upon  the  form  of  the  individual  arcs  and 
upon  the  character  of  their  vertical  sections.  Let  us  now 
consider  the  problems  of  mechanics  concerned  with  their 
development,  insofar  as  these  may  appear  from  their  de- 
sign or  pattern  as  a  whole  with  reference  to  the  ancient 
coigns  and  to  the  ocean  basins  upon  their  fronts.  If,  as  we 
have  assumed,  thrusts  proceed  outward  from  the  sinking 
sea-floors  toward  the  neighboring  coasts,  we  are  concerned 
with  the  way  in  which  the  higher  coasts  acting  in  the  manner 
of  a  resisting  mass  may  be  assumed  to  resolve  these  thrusts 
in  accord  with  the  known  laws  of  mechanics  applying  to 
beams  or  girders,  the  outer  shell  of  the  lithosphere  being 
looked  upon  as  capable  of  transmitting  the  stresses  to  con- 
siderable distances  under  these  conditions.  Obviously  the 
trend  of  a  coast  line  must  be  a  prime  determining  factor, 
and  the  results  may  differ  materially  for  different  outlines 
of  coast;  as,  for  example,  for  a  coast  reentrant,  for  a  straight 
coast  line,  and  for  a  salient.  The  case  of  a  salient  may 
further  offer  some  differences  according  as  it  is  obtuse  or 
acute. 

135 


136      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

An  attempt  has  been  made  in  Figs.  64-66  to  indicate  what 
are  these  essential  differences.  The  first  case,  that  of  a 
coast  reentrant  (Fig.  64),  illustrated  by  the  Eurasian  and 
North  American  coast  lines  where  they  approach  at  Bering 
Strait,  should  develop  an  anticlinal  arc  of  gentle  curvature 
everywhere  underturned  toward  the  front  in  the  normal 

RE-ENTRANT 


t 

FIG.  64. — Plan  of  an  arc  rising  in  a  reentrant  of  the  coast 

fashion.  A  straight  coast  line  may  be  assumed  to  be  much 
the  same  and  perhaps  with  even  simpler  curvature  and  with 
the  anticline  developing  underthrust  and  in  its  later  stages 
underslicing.  The  southern  Andes  appear  to  fairly  illustrate 
this  type. 

The  third  type  of  coast  line,  which  outlines  an  obtuse 
salient,  is  the  most  common  of  all  and  offers  no  essentially 


OBTUSE  5ALILNT 


1 

FIG.  65. — Plan  of  an  arc  rising  off  an  obtuse  salient  of  the  coast 

new  features  (Fig.  65).  The  greater  number  of  arcs  on  the 
eastern  border  of  Eurasia  forming  the  garlands  of  islands 
along  that  coast,  fall  within  this  class.  In  them,  so  far  as 
evidence  is  at  hand,  the  anticlines  are  underthrust  from  the 
front  and  are  of  the  same  asymmetrical  type,  whether 
viewed  from  the  middle  (frontal  position),  or  from  nearer 
the  ends — the  lateral  parts. 


THE   PATTERNS   OF  THE   FACIAL   WRINKLES         137 

On  the  other  hand,  a  sharp  salient  in  the  coast  line,  such 
as  is  offered  by  the  Malayan  peninsula  pinched  in  between 
the  Indian  and  Pacific  oceans,  obviously  offers  a  different 
problem;  for  the  two  sides  of  the  salient  are  each  capable 
of  resolving  the  thrusts  from  the  sea  in  an  individual  man- 
ner. The  attempt  to  derive  the  resulting  system  of  arcs 
for  this  type  of  coast  line,  as  has  been  done  in  fig.  66,  has 
gone  out  from  the  design  displayed  by  the  archipelagos 
about  the  Malayan  peninsula,  the  one  striking  example 
which  our  planet  affords  where  geosynclines  with  their 
lenses  of  sediments  line  both  flanks  of  the  salient. 
ACUTE:  SALIENT  ACUTE  SALIENT 

CARLYSTAOe  ,^_  /</  LATEST* 


FIG.  66. — Plans  of  arcs  in  two  successive  stages  formed  off  an  acute  salient 

of  the  coast 

An  attempt  to  give  anything  like  an  adequate  discussion 
of  this  area  must  be  reserved  for  another  place,  but  two  as- 
pects of  the  subject  may  be  touched  upon  here.  These  are, 
(A)  the  intersecting  knots,  or  centers  of  virgation  which 
have  developed,  and  (B)  the  back-folding  of  the  anticlines 
where  arcs  become  strongly  curved,  with  the  attendant  pro- 
duction of  back-troughs  instead  of  fore-troughs.  Viewed 
from  a  mechanical  standpoint  this  would  appear  to  be  a 
consequence  of  the  enhanced  rigidity  of  the  material  in- 
volved in  the  arc  through  pleating  (doubling)  of  the  strata. 
A  result  is  that  the  thrusts,  acting  in  larger  measure  upon 
the  lateral  portions,  overwhelm  them  at  the  same  time  that 
the  frontal  portion  of  the  arch  is  held  back  in  its  develop- 
ment. As  a  consequence  the  fold  continues  to  retain  its 


138      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 
I 


ii 


, 


fi 


)i 


$? 

fl 

;;L 


85 


THE   PATTERNS    OF   THE    FACIAL    WRINKLES         139 

early  stage  in  the  frontal  portion  and  no  fore-trough  de- 
velops, whereas  at  the  sides  the  fold  is  bent  backward,  or 
reversed,  and  a  back-trough  is  formed  behind  it — within  the 
arc  itself.  This  condition  probably  expresses  a  general  law, 
for  it  is  characteristic  of  all  the  more  sharply  curved  arcs 
of  the  Pacific  archipelagos  insofar  as  soundings  are  available 
to  permit  of  their  study.  Examples  are  furnished  by  the 
Banda,  Luisiade  and  Bismarck  arcs  (Fig.  67).  We  do  not 
find  in  this  the  evidence  that  tensional  stresses  are  produced 
in  the  arc  such  as  have  been  appealed  to  in  explanation  of 
the  straits  which  separate  the  islands  which  compose  the 
Malayan  arc.  We  believe,  rather,  that  the  fold  or  crease 
does  not  in  its  initial  stage  rise  uniformly  throughout,  but 
rather  as  a  series  of  domes  with  possible  laccolite  cores  be- 
neath, and  that  these  domes  only  in  a  later  stage  unite  into 
a  continuous  anticlinal  arc. 

Fortunately  we  have  the  opportunity  of  testing  the  de- 
formation within  arcs  which  have  long  been  above  the  sea 
and  in  which  the  folded  strata  lie  open  to  our  inspection. 
A  particularly  fortuitous  example  is  supplied  by  the  region 
in  the  states  of  Colorado,  Wyoming  and  Montana,  within 
which  strongly  curved  arcs  are  to  be  seen  in  the  positions 
in  which  they  formed  off  the  shore  of  the  former  Lara- 
mide  Ocean  to  the  eastward  of  the  Rocky  Mountains.  These 
island  arcs  rose  in  late  Cretaceous  and  early  Tertiary  time  as 
garlands  sweeping  along  the  shore  at  that  time  and  display- 
ing a  design  surprisingly  like  that  displayed  today  by  the 
map  of  the  eastern  coast  of  Eurasia  (Fig.  68).  Similar  to 
the  arcs  of  Japan  and  the  Philippines,  there  arose  in  this 
earlier  period  those  of  the  Belt  Mountains,  the  Big- 
horn and  the  Laramie  ranges  with  others  behind  them, 
and  with  a  knot  formed  at  the  junction  of  the  Uinta  and 
Wasatch  ranges;  while  out  in  front  other  arcs  were  just 
rising  above  the  surface  of  the  sea.  In  these  latter,  as  in  the 
smaller  islands  which  are  today  strung  along  the  western 
Pacific  boundary  as  pearls  are  strung  upon  a  thread,  we  find 


140      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


SECT  I  0  N  5 
ACROSS     A  RC 3 


BIG     HORN    ARC 

?.  m^fff\  [.'.  .  :'.-^»^ -^ 

BLACK    HILLS    DOME 


LARAMIC      ARC 


UINTA     ARC 


LEGEND 

c' ,'"_,.•' E  ocene    Deposits 


rura-  Trias  Outcrops 
*— 'Taramide    Arcs 

f  Granite     Cores 
Laccolitea 

POST   LARAMIE 
'     VOLCANO'ES 
O  Andesites  and  Rhyolites 
©  Basalts 
•   Feldspathoid  .Lavas 

UPPER    IN5ET 
*-S~    Tertiary  Arcs 
•  •    Active  Volcanoes 

LOWER    INSET 
*a*  Dakota  ^andstonepretaceous) 
—  Red  Beds  (Tnaasic) 
••:'  Pre-Mesozoic 


FIG.  68.— Sketch-map  of  the  arcs  formed  off  the  eastern  front  of  the  Rocky 
Mts.  (based  on  map  by  U.  S.  Geological  Survey) 


THE   PATTERNS    OF   THE    FACIAL    WRINKLES         141 

the  chains  of  laccolites  and  of  domed  strata  in  the  Little 
Rocky  Mountains  and  in  the  Black  Hills  with  its  extension 
in  rising  domes. 

For  each  of  these  two  classes,  the  continuous  curving  arc 
and  the  curved  chain  of  domes,  we  are  fortunate  in  having 
at  least  one  elaborate  series  of  geological  sections,  from 
which  it  is  possible  to  compare  the  frontal  with  the  lateral 


FIG.  69. — Series  of  sections  through  the  Bighorn  Range  and  showing  in- 
ward underthrust  in  the  center  combined  with  inward  overthrust  on 
the  flanks  (Darton's  sections) 

portions  in  respect  to  the  form  of  their  folds.  In  Fig.  69  are 
displayed  eleven  roughly  radial  sections  through  the  curv- 
ing arc  of  the  Bighorn  range,  within  which  sections  are  seen 
all  the  gradations  from  the  normal  anticline  in  the  frontal 
position  with  its  steeper  limb  directed  outward,  to  the 
strongly  back-folded  portions  in  lateral  position  within 
which  is  seen  the  evidence  of  an  inward  overthrust  from 
without. 

The  Black  Hills  group  of  domes  is  displayed  in  the  sec- 


142      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

tions  of  Fig.  70,  and  indicates  essentially  the  same  condi- 
tions— the  normal  underthrust  at  the  front  and  the  back- 
folding  from  inward  overthrust  within  the  lateral  portions 
of  this  strongly  curved  arc. 

From  all  these  examples  of  the  present  and  the  past  arc 
formations,  we  draw  the  conclusion  that  the  pleating  of 
competent  strata  so  stiffens  this  zone  in  which  the  crease  is 
made,  that  if  it  be  too  sharply  bent  in  its  plan  the  acquired 


FIG.  70. — Series  of  sections  showing  the  partly  buried  arc  of  the  Black 
Hills  with  its  inward  underthrust  in  the  center  combined  with  inward 
overthrust  on  the  flanks  (Barton's  sections) 

rigidity  of  the  crease  asserts  itself  in  mutual  reactions  of  its 
frontal  portion  upon  its  extremities  and  vice  versa. 

The  Cordilleran  system  of  mountains  in  North  America 
offers  an  interesting  example  of  the  direct  relationship  of 
the  competence  of  the  developing  anticline  to  the  extrava- 
sation of  lavas  in  its  vicinity.  In  that  portion  of  the  system 
which  is  included  in  Colorado  (see  Fig.  68)  and  has  there 
yielded  a  series  of  arcs  within  which  the  folds  are  of  fairly 
symmetrical  form,  the  development  of  laccolitic  cores  and 
the  extravasation  of  lava  have  both  taken  place  upon  a 


THE   PATTERNS    OF   THE    FACIAL    WRINKLES         143 

grand  scale;  whereas  in  the  zone  to  the  northward,  where 
instead  of  taking  up  the  slack  of  the  compressed  strata  by  a 
series  of  open  and  therefore  competent  folds,  this  adjust- 
ment has  all  been  concentrated  within  a  relatively  narrow 
zone  of  incompetent  underthrust  and  undersliced  folds  (see 
sections  of  Fig.  59,  p.  130,  neither  laccolites  nor  volcanic 
extravasations  has  resulted.  This  appears  to  correspond  to 
a  rather  general  rule  which  applies  to  underthrust  and 
undersliced  folds  and  must  be  looked  upon  as  valuable  con- 
firmatory evidence  in  support  of  the  view  of  the  origin  of 
lava  from  the  fusion  of  shales.  This  difference  in  character 
of  the  folding  Dr.  R.  T.  Chamberlin  has  recently  sought  to 


!U!l! 

ll;||    J« 

<O                      ^ 

Sea.      Level      

H£2:v        '          J           *  -        ;- 

Sea  Floor 
acting  aa  Girder 


EIG.  71.  —  Schematic  diagram  to  show  plan  of  formation  of  rising  arc  at 
the  front  of  a  lense  of  sediments 

explain  through  a  difference  in  character  of  the  involved 
strata. 

If  we  accept  the  general  plan  of  development  of  arcs  with 
respect  to  the  form  of  the  coast  line  as  this  was  set  forth  in 
figs.  64-66,  there  still  remains  the  interesting  question  con- 
cerning the  exact  places  where  anticlines  will  rise  and  form 
the  individual  arcs.  Why,  for  example,  does  the  Aleutian 
arc  rise  just  where  it  appears  upon  the  map,  rather  than 
either  farther  out  from  or  nearer  to  the  apex  of  the  reentrant 
at  Bering  Strait?  It  seems  highly  probable  that  the  point 
of  initiation  of  an  anticline  is  to  a  large  degree  determined 
by  the  initial  dip  of  the  lenses  of  deposits  as  these  bow  up- 
ward in  the  zone  of  the  geosyncline  toward  the  margin  of 
the  continental  shelf  (Fig.  71).  A  stiff  rod,  otherwise 
straight,  will  buckle  under  compression  from  its  two  ends 


144      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

at  the  place  where  it  has  an  initial  bend  from  its  straight 
axis,  however  slight  in  amount  this  may  be. 

Behind  the  arcs,  wherever  they  rise  as  a  festoon  of  off- 
shore islands,  sedimentation  will  take  place  and  eventually 
close  up  the  sea  which  is  thus  initiated.  Such  a  process  of 
sedimentation  is  today  going  on  most  rapidly  in  the  Yellow 
Sea  behind  the  Riukiu  arc  on  the  China  coast,  into  which 
sea  the  Yellow  River  discharges  its  great  burden  of  sedi- 
ment. 

In  the  interesting  experiments  by  Willis  which  have  al- 
ready been  frequently  referred  to,  it  was  shown  that  after 
the  first  anticline  to  develop  had  lost  its  competence 
through  underturning,  a  second  anticline  developed  behind 
it,  and  in  turn  a  third  and  perhaps  a  fourth,  but  of  smaller 
size  as  they  recede  from  the  original  arc.  It  is  evident  from 
the  study  of  anticlines  formed  in  the  Malayan  archipelago 
that  some  such  order  is  characteristic  of  rock  formations, 
though  later  subordinate  anticlines  are  probably  also  formed 
in  front  of  the  initial  arch.  In  the  Willis  experiments  the 
active  thrust  was  transmitted  through  a  rigid  piston  which, 
so  far  as  the  experiments  were  concerned,  might  be  re- 
garded as  of  infinite  rigidity.  In  the  case  of  anticlines  de- 
veloped in  strata,  the  active  thrust  is  communicated  through 
the  competent  stratum  itself,  and  the  study  of  fore-troughs 
in  the  Pacific  region  indicates  pretty  clearly  that  after  an 
initial  and  essentially  major  anticline  has  been  pushed  up, 
at  least  one  smaller  anticline  is  apt  to  develop  along  its 
front  and  produce  what  we  have  called  a  crown-deep  or 
crown-trough,  illustrated  particularly  well  by  the  sea-floor 
at  the  front  of  Java  and  Bali  (Fig.  50,  p.  121). 

If  we  now  devote  our  attention  to  the  general  plan  of  the 
earth's  face  (see  Frontispiece),  we  perceive  at  once  the  re- 
lationship which  the  later  formed  arcs  bear  to  the  frame- 
work of  the  structure  displayed  in  the  ancient  coigns.  It  is 
about  these  relatively  rigid  foundation  stones  of  the  litho- 
sphere  that  the  structure  is  laid.  The  coigns  within  the 


THE   PATTERNS    OF   THE    FACIAL    WRINKLES         145 

southern  hemisphere  have  been  relatively  less  immobile 
than  those  of  the  northern  hemisphere,  as  is  indicated  by 
the  fact  that  they  have  been  partially  covered  by  the  Cre- 
taceous deposits.  It  is  only  between  them,  where  the  thick 
deposits  of  the  Mesozoic  geosyncline  stretch  out  along  the 
twin  plane  of  the  earth,  that  the  arcs  have  been  able  to  de- 
velop in  long  and  narrow  loops.  This  girdle  of  the  earth  is 
now  represented  by  the  arcuate  zone  joining  the  Caribbean 
Sea  in  the  western  hemisphere  to  the  Mediterranean  Sea 
of  the  western  portion  of  the  eastern  hemisphere  extended 
to  the  Malayan  zone  to  the  southeast.  A  somewhat  similar 
belt  pushes  out  eastward  within  the  area  between  Pata- 
gonia and  the  Antarctic  continent  in  the  direction  of  another 
portion  of  the  known  Mesozoic  geosyncline  following  the 
East  African  littoral. 

The  order  of  development  of  these  wrinkles  in  the  earth's 
face  is  shown  both  by  the  geologic  age  of  the  formations 
which  have  become  involved  in  the  folding  process,  and  by 
the  stage  of  subsequent  denudation,  to  have  been  from  the 
coigns  outward.  The  outermost  arcs  of  the  series  are  even 
today  in  process  of  rapid  elevation,  as  is  attested  by  the 
jolting  movements  manifested  to  our  senses  as  earthquakes, 
and  by  the  fusion  and  exudation  of  molten  rock  at  the  vents 
of  the  active  volcanoes.- 

As  we  go  back  from  these  later  arcs  of  the  outer  series, 
we  pass  into  arcs  of  mountains  which  today  rise  to  the 
loftiest  heights  that  are  anywhere  known,  but  with  the 
quaking  renewals  of  the  erecting  process  now  relatively  in- 
frequent. The  process  is  thus  in  this  zone  nearing  its  end, 
and  the  tearing  down  agencies  are  doing  their  work  of  level- 
ling out  and  slowly  erasing  these  wrinkles  from  the  earth's 
face. 

Still  farther  back  toward  the  ancient  and  long  stabilized 
Angara  shield,  the  work  of  levelling  down  has  been  already 
accomplished,  and  the  former  ranges  have  left  us  little  save 
their  roots.  These  inner  arcs  we  note  also  have  a  simpler 


146      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

and  straighter  course,  and  about  them  the  outer  arcs  have 
been  laid  on  in  the  form  of  festoons  ever  more  differentiated 
in  character  as  we  pass  toward  the  outer  marginal  series. 

The  inner  and  earlier  arcs  trending  in  roughly  east-west 
direction  south  of  the  ancient  coign  of  Angara  Land,  from 
their  age  must  have  been  formed  when  the  Sea  of  Tethys 
stretched  in  the  same  direction  to  the  south  of  them,  so  that 
the  underthrust  would  be  to  the  northward.  The  later  arcs 
with  their  more  intricate  pattern  of  festoons  developed  in 
the  deposits  of  the  Tethys  Sea  and  rose  with  the  founder- 
ing of  the  portions  of  the  Gondwana  continent  which  had 
formed  its  southern  border,  and  which  after  the  foundering 
formed  the  bed  of  the  Indian  Ocean. 

The  strikingly  compressed  arcs  which  extend  the  Hima- 
layan highland  southeastward  in  the  direction  of  the  coign 
of  Australia,  represent  a  much  widened  area  of  Mesozoic 
deposits  where  the  two  great  zones  of  geosynclines  approach 
and  intersect,  and  where  these  were  caught  in  the  vise  be- 
tween the  thrusts  of  the  sinking  basin  of  the  Pacific  and  that 
of  the  Indian  Ocean,  while  protection  was  being  furnished 
at  the  southern  end  by  the  shield  of  Australia.  (Fig.  72).  To 
the  eastward  of  Australia  the  extension  of  this  series  seems 
to  have  been  crowded  against  the  Australian  coign  by  the 
thrust  from  the  broad  sinking  area  of  the  Pacific  (see 
frontispiece) . 

An  even  more  striking  instance  of  compression  of  arcs 
is  furnished  by  the  Andean  system  of  South  America  which 
was  compressed  both  from  the  east  and  from  the  west.  This 
intense  compression  is  reflected  in  the  closely  squeezed  and 
nearly  vertical  isoclinal  folds  with  their  flattened  laccolitic 
cores,  so  well  brought  out  in  Steinmann's  generalized  sec- 
tion (Fig.  20,  p.  58).  Around  the  northern  end  of  the 
Brazilian  coign  in  Peru,  this  compressed  series  of  arcs  opens 
out  in  most  strikingly  schematic  fashion  as  the  sheaf  of 
mountain  chains  which  pushes  northeastward  into  the  is- 
land arcs  of  the  Antilles  and  northwestward  as  a  long  ex- 


THE   PATTERNS   OF   THE    FACIAL    WRINKLES         147 


tended  arc  whose  course  may  be  faintly  followed  by  the  vol- 
canic islands  off  the  west  coast  of  South  America  (see 
frontispiece).  At  the  compound  Peruvian  knot  there  is 
found  the  chief  development  of  volcanic  energy,  as  is  well 
brought  out  in  Karsten's  geological  map  (Fig.  73),  to  which 


Pre-Cambrian  Arcs 

Arcs  formed  at  close  of 

Paleozoic  Era 
Arcs  -formed  at  close  of 

Mesozoic  Era 
•••  Volcanoes 


FIG.  72. — Sketch  map  to  show  the  arcs  of  southeastern  Asia 

we  have  added  only  the  heavy  lines  to  show  the  approximate 
courses  of  the  several  arcs. 

The  best  indication  of  the  rising  arcs  upon  the  floors  of 
the  oceans  is  a  curving  ridge  or  curving  chain  of  islands  with 
a  definite  fore-trough  at  its  front.  It  is  only  after  these 
features  have  developed  sharply  inclined  slopes  that  the 
volcanic  activity  is  likely  to  be  observed  as  an  important 


148      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

confirming  indication  that  an  arc  is  forming  in  this  position. 
In  the  earlier  stages  of  arc  formation  the  indication  of  its 
location  will  naturally  be  relatively  slight.  We  are  there- 
fore in  great  need, of  additional  soundings  from  the  ocean- 
floor  in  significant  localities  where  the  possibility  of  an  arc 
is  indicated,  and  until  such  soundings  have  been  made  no 
clear  statements  can  be  offered  for  certain  sections  of  the 
ocean  areas.  The  rich  results  of  the  soundings  made  by 


LEGEND 

rv\  Ttrtiory  and  Later  Arc  3 
ronite,Gne>53,Schijt3.ftc 
^2  Cretaceous 
I       I  Ternary  and  Quartenory 
F^l  Volcanic  Rocks 
d3  Volcanoes 


FIG.  73.— Virgation  of  the  arcs  of  the  Andes  at'  the  Peruvian  knots  (after 
Karsten's  geological  map) 

—.- 

the  "Nero/5  "Planet"  and  "Siboga"  expeditions  should  leave 
us  in  no  doubt  as  to  the  real  importance  of  continuing  such 
investigations.  There  is  the  most  urgent  need  of  a  new 
"Challenger"  expedition  for  investigation  of  the  Pacific  area, 
which  is  to-day  a  region  scientifically  almost  unknown. 

The  Central  Atlantic  area  shows  upon  the  basis  of  the 
few  soundings  a  long  S-shaped  ridge  which  passes  through 
the  Azores  and  Madeira,  and  which  may  indicate  the  be- 
ginnings of  a  zone  of  arcuate  folding.  The  evidence  is, 


THE   PATTERNS    OF   THE    FACIAL    WRINKLES          149 

however,  too  scanty  to  make  prediction  safe  at  the  present 
time. 

In  eastern  Africa  and  in  the  neighboring  portions  of  the 
Indian  Ocean  there  is  a  large  area  within  which  arcuate 
folding  would  appear  to  be  combined  with  faulting,  but, 
unlike  the  Eurasian  continent  the  folding  is  here  subordi- 
nate to  the  faulting  process.  Within  the  western  portion 
of  this  region  upon  the  continent  of  Africa  we  find  some  of 
the  most  stupendous  fault  displacements  that  are  anywhere 
known,  and  these  are  so  associated  as  to  outline  great 
trenches  or  rifts  of  which  the  more  remarkable  are  indicated 
by  the  position  of  lakes  Tanganika,  Albert  Nyanza,  and 
Nyassa. 

The  view  of  Suess  that  faulting  alone  has  produced  the 
structures  found  in  the  rocks  of  this  portion  of  the  African 
continent,  has  been  opposed  by  de  Lapparent,  who  regards 
the  great  rifts  which  outline  the  western  boundary  of  the 
area  as  the  broken  western  limb  of  a  sharp  anticline  with  its 
steeper  side  towards  the  west.  This  view  seems  to  be  sup- 
ported also  by  Abendanon. 

Gregory,  after  revisiting  the  scene  of  his  earlier  studies  in 
the  Rift  Valley  region,  says  in  a  recent  paper,  "The  first 
stage  in  the  formation  of  the  Rift  Valley  was  the  elevation 
of  the  country  along  it  into  a  low,  broad  arch,  which  would 
have  formed  a  belt  of  down-like  highlands,  ranging  across 
British  East  Africa  from  north  to  south.  *  *  *  The  more 
active  earth  movements  in  late  Cretaceous  times  and  the 
foundering  of  the  floor  of  the  Indian  Ocean  between  East 
Africa  and  India,  led  to  volcanic  outbreaks  in  western  India 
on  a  colossal  scale.  *  *  *  If  the  formation  of  the  Arabian 
Sea  led  to  such  violent  volcanic  activity  in  India  it  is 
natural  to  expect  some  corresponding  events  in  East  Africa, 
on  the  other  side  of  the  foundered  land." 

Suess  had  indeed  noted  the  upswelling  of  the  Rift  Valley 
borders,  and  Jaeger  makes  this  arching  of  the  strata  precede 
the  faulting. 


150      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

In  the  area  of  the  western  Indian  Ocean  adjacent  to  the 
East  African  region  under  consideration,  the  courses  of  the 
submerged  ridges  strongly  suggest  that  these  are  the  initial 
stages  in  the  formation  of  folded  arcs,  and  in  part  this  view 
is  confirmed  by  the  active  volcanoes.  Though  we  must 
await  the  results  of  future  soundings  to  settle  this  question, 
the  arcs  suggested  by  the  submerged  ridges  have  been  out- 
lined upon  the  map  in  dotted  lines. 

Perhaps  the  most  interesting,  but  also  the  most  complex, 
of  all  the  regions  of  arcuate  mountains,  are  those  which  fol- 
low the  ancient  twin  plane  of  the  earth  and  include  the 
American  and  the  European  Mediterraneans  as  well  as  the 
Malayan  and  Australasian  regions;  but  the  discussion  of 
these  areas  must  be  reserved  for  another  place. 

Intermontane  valleys  along  the  western  base  of  the  Andes 
and  its  extensions  into  North  America  take  the  place  of  the 
fore-deeps  characteristic  of  the  western  margin  of  the 
Pacific  Ocean.  They  appear  to  be  in  part  at  least  fault 
rifts  differing  in  degree  rather  than  in  kind  from  the  African 
rift  valleys. 

LITERATURE 

H.  A.  BROUWER.    On  the  Tectonics  of  the  Eastern  Moluccas,  Kon.  Akad. 

v.  Wetensch.,  Amsterdam,  vol.  19,  no.  2,  pp.  197-209. 
H.  A.  BROUWER.    Fractures  and  faults  near  the  surface  of  moving  geoanti- 

clines,  I,  ibid.,  vol.  23,  no.  4,  pp.  1-7  (reprint). 
G.  A.  F.  MOLENGRAAFF.     Modern  Deep-Sea  Research  in  the  East  Indian 

Archipelago,  Geogr.  Jour.,  vol.  57,  1921,  pp.  95-121. 
E.  C.  ABENDANON.    Die  Grossfalten  der  Erdrinde,  Leiden,  1914,  pp.  115- 

130. 
ALEX.  SUPAN.     Die  Bodenform  des  Weltmeeres,  Pet.  Mitt.,  vol.  45,  1899, 

pp.  177-188,  pi.  12. 

ALEX.  SUPAN.    Die  Sunda  Graben,  ibid.,  vol.  53,  1907,  pp.  70-71,  pi.  6. 
E.    HORN.    Ueber   die    geologische    Bedeutung   der   Tiefseegraben,    Geol. 

Rundschau.,  vol.  5,  1914,  pp.  422-448. 
HANS  STILLE.     Alte  und  .iunge  Saumtiefen,  Nachr.  d.  K.  Gesell.  d.  Wiss. 

z.  Gottingen,  Math.-phys.  Kl.,  1919,  pp.  1-36  (reprint). 
GROLL.     Tiefenkarten   der  Ozeane   mit  Erlaiiterungen,  Veroff.  d.  Inst.   f. 

Meereskunde,  Berlin,  1912,  N.  F. 
A.   DE   LAPPARENT.     Soulevements   et   affaisements,   Revue    des   Questions 

Scientifiques,  vol.  14,  1898,  pp.  22-24. 

HERMANN   BERGHAUS.    Atlas  der   Geologie,  Justus  Perthes,  Gotha,  1892. 
HERMANN  KARSTENS.     Geologie  de  Pancienne  Columbie  Bolivienne,  Vene- 
zuela, nouvelle  Grenade  et  Equador,  Berlin,  1886,  map. 
J.  B.  WOODWORTH.    Geological  expedition  to  Brazil  and  Chile,  1908-1909, 

Bull.  Mus.  Comp.  Zool.,  vol.  16,  no.  1,  p.  117. 


THE   PATTERNS    OF   THE    FACIAL    WRINKLES         151 

W.   D.   SMITH.    The   Philippine   Islands,   Handb.   d.   reg.    Geol.,   vol.   6, 

Abt.  5,  p.  23,  fig.  5. 

W.  H.  HOBBS.    Earth  Features  and  their  Meaning,  1912,  pp.  436-439. 
W.   H.    HOBBS.    Mechanics   of   Formation   of   Arcuate    Mountains,   Jour. 

Geol.,  vol.  22,  pp.  77-82. 
J.  W.  GREGORY.    The  geological  history  of  the  Rift  Valley,  Jour.  East. 

Africa  and  Uganda  Nat.  Hist.  Soc.,  vol.  6,  1920,  pp.  429-440.    See  also 

The  African  rift  valleys,  Geogr.  Jour.,  July,  1920,  pp.  13-47. 


CHAPTER  XII 

THE  DESIGN  OF  THE  FRACTURE  MARQUETRY 

OF  the  two  contrasted  aspects  of  the  earth's  face  to  which 
attention  was  first  directed  by  Suess,  we  have  now  seen  that 
the  one  exemplified  by  the  greater  part  of  the  continent  of 
Eurasia  with  its  extensions  to  the  southeastward,  is  char- 
acterized by  curved  wrinklings  in  the  rock  strata  appearing 
as  great  earth  welts  which  rise  above  the  general  surface  in 
a  pattern  of  concentric  arcs  festooned  about  the  rigid  coigns 
of  the  earlier  eras.  The  origin  of  these  arcs  is  clearly  bound 
up  with  the  folding  processes  and  believed  to  be  due  to  the 
continued  contraction  of  the  earth's  core.  Adjustments  on 
planes  of  fracture  play  a  secondary  rather  than  a  primary 
role  in  the  formation  of  these  arcs. 

We  turn  now  to  the  contrasted  facial  expression  of  the 
earth  exemplified  by  the  greater  portion  of  the  continent  of 
Africa,  a  large  part  of  the  Great  Basin  of  the  western  United 
States,  and  other  regions,  where  displacements  by  block 
faulting  rather  than  folding  have  been  mainly  responsible 
for  the  pattern  of  the  surface  configuration.  The  vast  area 
of  Africa  has  been  as  yet  little  developed  commercially,  if 
we  compare  it  with  other  portions  of  the  world,  and  our 
knowledge  of  its  geology  has  been  correspondingly  meager. 
In  the  Great  Basin  region  of  the  western  United  States,  the 
early  reports  by  the  geological  pioneers  of  the  region,  King, 
Powell,  Button,  Gilbert  and  Russell,  all  agreed  in  ascribing 
the  major  features  of  the  relief  to  fault  displacements  in 
which  north  and  south  fractures  were  dominant.  Suess  in 
his  discussion  of  the  great  faults  of  East  Central  Africa  has 
shown  that  in  that  region  also  the  north-south  displace- 

152 


THE  DESIGN  OF  THE  FRACTURE  MARQUETRY        153 

ments  are  much  the  most  prominent.  Later  studies  by 
Passarge  and  by  Simmer  have  shown,  however,  that 
these  African  meridional  displacements  constitute  but  one 
series  within  a  fracture  pattern  composed  mainly  from  four 
series,  and  that  this  fault  pattern  divides  up  the  entire 
African  continent  into  a  fault  mosaic.  The  four  directions 
of  the  fault  series  are  approximately  north-south,  east-west, 
northwest-southeast  and  northeast-southwest.  Not  all 
these  directions  are  found  developed  in  each  part  of  the 
region.  Thus  in  the  region  of  the  great  rifts  of  the  east- 
central  portions  of  the  continent,  the  east-west  direction 
is  certainly  less  prominently  brought  out  than  the  three 
others;  and,  as  Suess  first  pointed  out,  the  direction  of  the 
meridian  is  the  one  most  strikingly  manifested,  and  notably 
in  the  course  of  the  Nile  (Fig.  74,  center  map). 

A  study  by  Spurr  of  the  basin  ranges  in  the  western 
United  States  has  likewise  shown  that  the  same  four  di- 
rections— those  of  the  cardinal  points  and  of  their  inter- 
mediate bisecting  directions — have  played  the  major  role 
in  determining  the  pattern  of  the  displacements.  In  the 
earlier  studies  of  the  Great  Basin  region  it  was  assumed  that 
the  structure  was  entirely  due  to  fault  displacements,  but 
Spurr  was  the  first  to  show  that  while  faults  may  have  de- 
termined the  pattern,  folding  has  also  played  an  important 
role,  and  the  later  studies  of  the  Basin  ranges,  as  these  have 
gained  importance  in  mining  operations,  have  supplied 
ample  confirmation  for  this  view  (see  Fig.  62,  p.  133). 

It  seems  to  be  no  less  clear  that  within  the  areas  where 
arcuate  folding  is  the  dominant  structure  and  has  imparted 
its  character  to  the  surface  configuration,  faulting  has  by  no 
means  been  absent  or  without  its  effect  upon  the  topog- 
raphy. One  could  hardly  hope  to  find  a  district  where 
the  fold  structures  are  more  strikingly  displayed  in  the  re- 
lief than  in  the  spider-like  outlines  of  the  island  of  Celebes; 
yet  the  fracture  pattern  is  also  marked,  and  the  faults  are 
in  evidence  in  a  characteristic  network  of  which  the  mer- 


iv.l.m, 


FIG.  74. — Samples  of  the  pattern  of  the  fracture  network  of  the  earth.  (A), 
the  network  itself;  1,  rectangular  system  of  master  joints;  3,  escarpe- 
ment  due  to  master  joints  in  sub-equally  spaced  series  near  Cayuga 
Lake,  N.  Y.;  4,  joint  system  with  composite  groups  and  a  patterned 
fault  system  evolved  from  the  joint  system  by  displacement,  Nor- 
wegian coast;  9,  ground  plan  of  the  system  with  pattern  changing  its 
position;  10,  fractures  produced  in  block  of  an  elastic  substance 
(moulder's  wax)  by  compression  from  the  ends.  (B),  Imprint  of  the 
fracture  pattern  in  the  earth's  face;  2,  patterned  drainage  lines  of  an 
area  in  Connecticut;  5,  the  gigantic  fault-rifts  of  East  Central  Africa; 
6,  the  fracture  controlled  course  of  a  canyon  in  Swedish  Lapland;  7, 
" Checker-board"  drainage  of  Western  Ontario;  8,  map  of  the  Batoka 
Gorge  below  the  Victoria  Falls,  Rhodesia,  controlled  by  the  fracture 
network;  11,  drainage  of  the  "dolomites"  of  the  Tyrol  controlled  by 
the  fracture  pattern 

154 


THE  DESIGN  OF  THE  FRACTURE  MARQUETRY        155 


idional  and  east-west  series  dominate  (Fig.  75).  Brouwer, 
who  has  done  so  much  to  decipher  the  intricate  fold  struc- 
tures within  the  region  of  the  Dutch  East  Indies,  takes  ac- 


ArcsCheavy  lines)and fracture  networkdight  lines) of  CelebesCAhlburg) 

FIG.  75.— Map  showing  the  fracture  network  of  the  Island  of  Celebes  in 
its  relation  to  the  fold  lines  (after  Ahlberg) 

count  also  of  the  subordinate  fault  structures  which  have 
contributed  their  part. 

In  1904  and  1905,  on  the  basis  of  a  survey  of  the  field  of 
tectonics,  the  present  writer  brought  out  the  fact  that  there 
is  evidence  for  a  pattern  of  fractures,  made  up  in  part  of 


156      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

joint  fissures  and  in  part  of  displacements  along  them — 
faults;  and  that  this  pattern  of  fractures  is  at  least  con- 
tinental in  its  extent  and  probably  world-wide.  Of  this 
pattern  the  directions  which  have  determined  the  mosaic  of 
the  African  continent — the  two  cardinal  directions  and  the 
two  intermediate  bisecting  ones — hold  good  also  through- 


/  Joint  System 

FIG.  76. — Sketch-map  showing  the  prevailing  direction  of  joint  fissures  in 
Southern  South  America  (after  Windhausen) 

out.  The  results  of  this  study  were  first  published  in  the 
Transactions  of  the  Wisconsin  Academy  of  Science,  Arts  and 
Letters,  and  later  with  greater  thoroughness  in  the  Bulletin 
of  the  Geological  Society  of  America  and  in  the  Comptes 
Rendus  of  the  International  Geological  Congress  held  at  St. 
Louis  in  1906. 

A  few  selected  examples  to  illustrate  the  manner  of  ex- 
pression of  the  fracture  pattern  in  the  earth  relief  are 


THE  DESIGN  OF  THE  FRACTURE  MARQUETRY        157 

brought  out  by  the  river  networks  included  in  Fig.  74.  It 
was  of  considerable  interest  to  find  that  this  fracture  pattern 
of  the  earth  largely  ignores  geological  formation  boundaries 
and  passes  from  one  district  to  another  apparently  little  af- 
fected in  its  orientation  by  the  distribution  of  geological 
formations.  Quite  recently  this  fracture  pattern  has  been 
found  to  apply  to  a  considerable  portion  of  the  South 
American  continent  (Fig.  76). 

Inasmuch  as  in  both  of  the  contrasted  regions  of  the 
globe  there  is  found  the  evidence  of  both  fault  and  fold 
structures  within  the  same  strata,  it  becomes  a  matter 
rather  of  determining  which  one  dominates  in  the  major  de- 
formations. Such  a  result  is,  moreover,  in  support  of  the 


x>   e/^r  :: — 


FIG.  77. — Schematic  diagram  to  illustrate  the  relation  of  the  fractures  to 
the  growing  anticline 

view  that  the  processes  of  folding  and  faulting  go  on  simul- 
taneously within  the  same  strata,  rather  than  in  separate 
depth  zones,  as  called  for  by  the  theory  of  the  zones  of 
fracture  and  flow.  If,  as  we  have  claimed,  the  element  of 
time  is  really  the  vital  factor  in  the  problem,  there  should 
be  greatly  subordinated  fault  structures  wherever  under 
pronounced  tangential  compression  the  adjustments  within 
the  strata  go  on  slowly ;  but  the  fault  displacements,  should, 
on  the  other  hand,  be  the  dominating  ones  in  those  cases 
where  deformation  is  especially  rapid,  for  in  such  cases  the 
adjustments  may  take  place  within  the  relaxation  time  of 
the  strata.  At  greater  depths,  and  hence  under  greater 
loads,  the  plasticity  of  the  rocks  will  be  correspondingly 
augmented,  and  folding  is  likely  to  be  the  dominant  type 
of  deformation.  Small  faults  will  generally  be  in  evidence 
at  the  surface  with  the  measure  of  the  throws  on  the  dis- 


158      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

placements  of  the  fracture  pattern  distributed  as  required 
by  the  developing  fold  beneath  (Fig.  77).  The  forms  of  the 
reefcaps  in  the  islands  of  the  Dutch  East  Indies,  as  these 
have  been  described  by  Brouwer,  are  wholly  in  accord  with 
this  interpretation  that  the  mosaic  of  blocks  within  the 
fracture  pattern  undergoes  adjustments  of  such  a  nature  as 
to  accomplish  a  portion  of  the  bending  necessary  to  bring 
the  surface  strata  into  the  form  of  the  developing  fold. 

LITERATURE 

W.  H.  HOBBS.    The  Newark  system  of  the  Pomperaug  Valley,  Connecticut, 

21st.  Ann.  Rept.,  U.  S.  Geol.  Surv.,  part  3,  1901,  pp.  162,  pis.  17. 
W.  H.  HOBBS.     The  lineaments  of  the  Atlantic  border  region,  Bull.  Geol. 

Soc.  Am.,  vol.  15,  1904,  pp.  483-506,  pis.  45-47;  also  Comptes  Rendus 

Congres  Geol.  Intern.,  1906,  pp.  193-203. 
W.  H.  HOBBS.    The  correlation  of  fracture  systems  and  the  evidences  for 

planetary  dislocations  within  the  earth's  crust,  Trans.  Wis.  Acad.  Sci., 

vol.  15,  1905,  pp.  15-29. 
W.  H.  HOBBS.    Repeating  patterns  in  the  relief  and  in  the  structure  of 

the  land,  Bull.  Geol.  Soc.  Am.,  vol.  22,  1911,  pp.  123-176,  pis.  7-13.    See 

also  Earth  Features  and  their  Meaning,  chap.  17. 
J.  P.  IDDINGS.    A  fracture  valley  system,  Jour.  Geol.,  vol.  12,  1904,  pp.  94- 

105,  pi. 
L.  V.  PIRSSON.    Crustal  warping  in  the  Temagami-Temiskaming  district, 

Ontario,  Am.  Jour.  Sci.,  vol.  30,  1910,  pp.  25-32. 
TH.  KJERULF.    Geologie  von  Norwegen   (authorized  German  edition  by 

Gurlt  of  Bonn),  1880,  p.  332,  fig.  279. 

A.  DAUBREE.     Geologie  Experimental,  Paris,  1879,  pp.  332-375,  pis.  3-6. 
E.  C.  HARDER.    The  joint  system  in  the  rocks  of  southwestern  Wisconsin 

and  its  relation  to  the  drainage  network,  Bull.  Univ.  Wis.,  Sci.  Ser., 

vol.  3,  no.  5,  1906,  pp.  207-246,  pis.  1-10. 
A.  WINDHAUSEN.     The  problem  of  the  Cretaceous-Tertiary  boundary  in 

South  America  and  the  statigraphic  position  of  the  San-Jorge  forma- 
tion in  Patagonia,  Am.  Jour.  Sci.,  vol.  44,  1918,  fig.  3,  p.  33. 
SIEGFRIED  PASSARGE.    Die  Kalahari,  Berlin,  1904,  pp.  79-80  (C.  Die  Grund- 

linien  im  tektonischen  Aufbau  Siidafrikas). 
HANS    SIMMER.    Der   aktive   Vulkanismus   auf   dem    afrikanischen    Fest- 

lande  urid  den  afrikanischen  Inseln,  Gunther's  Munchener  Geograph- 

ischer  Studien,  no.   18,  1906,  p.  24. 
J.  E.  SPURR.    Origin  and  structure  of  the  Basin  Ranges,  Bull.  Geol.  Soc. 

Am.,  vol.  12,  1901,  p.  241. 
JOHANNES    AHLBURG.    Versuch    einer    geologische    Darstellung    der   Insel 

Celebes,  Geol.  u.  Pal.  Abh.,  vol.   16   (N.F,  vol.  12),   1913-1914,  pp. 

172,  pi.  8. 
J.    G.    LIND.    Geologische    Untersuchung    der    Beziehung    zwischen    Ges- 

teinspalten,    der    Tektonik    und    dem    hydrographischen    Netzes    des 

Gebirges  zu  Heidelberg,  Verh.  d.  naturh.  medicin.    Vereins  z.  Heidel- 
berg, N.  F.,  vol.  11,  1910,  pp.  7-45,  map. 


CHAPTER  XIII 

LAVA  COMPOSITION  IN  RELATION  TO  EARTH 
PHYSIOGNOMY 

IT  is  only  in  comparatively  recent  times  that  fruitful  at- 
tempts have  been  made  in  a  large  way  to  correlate  the  dis- 
tribution of  the  igneous  rock  types  in  respect  to  geological 
provinces  of  the  earth.  The  earlier  studies  placed  all  the 
stress  upon  supposed  fundamental  differences  being  de- 
pendent upon  the  geological  age  of  the  magma;  while  the 
later  ones,  made  after  the  age  differences  had  been  found  to 
be  less  important  than  had  been  believed,  upon  the  depth 
below  the  earth's  surface  at  which  the  rocks  had  been  con- 
solidated. 

The  conception  of  petrographical  provinces  dates  from 
1886,  and  the  rocks  within  these  provinces  were  recognized 
as  possessing  certain  family  relationships  to  which  the  term 
consanguinity  was  aptly  applied.  One  of  the  most  striking 
examples  of  consanguinity  was  early  discovered  in  the  vol- 
canic rocks  of  the  American  Cordillera,  and  particularly  in 
its  southern  section,  the  Andean  system  of  South  America. 
The  prevailing  type  of  lava  found  within  this  elongated  belt 
is  a  rock  of  notably  intermediate  chemical  composition — 
neither  very  rich  nor  very  poor  in  its  content  of  silica,  and 
one  characterized  by  an  arrangement  of  its  elongated 
crystals  in  waving  flow  lines,  a  texture  known  as  andesitic 
structure. 

By  Iddings  it  was  pointed  out  that  on  the  American  con- 
tinent the  sub-alkaline  group  of  the  andesites  in  the  Cor- 
dillera was  sharply  differentiated  from  an  alkaline  group 
which  characterized  the  areas  to  the  eastward  both  in  the 
United  States  and  Canada  and  in  Brazil.  Harker  in  1896 

159 


160      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

correlated  these  separated  provinces  for  the  sub-alkaline 
and  the  alkaline  groups  with  the  Pacific  and  Atlantic  types 
of  coast  line  as  these  had  been  differentiated  by  Suess,  and 
from  this  correlation  appears  to  have  been  derived  the  use 
of  the  terms  "Pacific"  and  "Atlantic"  lava  types. 

In  contrast  both  to  the  constancy  of  character  and  to  the 
near-average  rock  composition  within  such  a  province  as 
that  of  the  Andes,  the  other  type  of  magmas  quite  generally 
represented  greater  variations  both  among  themselves  and 
in  departures  from  the  average  magma.  As  an  example  we 
may  take  the  province  of  the  Bohemian  Mittelgebirge,  and 
the  contrast  which  it  affords  with  that  of  the  Andes  was  one 
of  the  first  to  be  studied  in  detail.  The  types  of  rock  char- 
acteristic of  these  areas  are  the  following: 

Bohemian   Mittelgebirge  Andes 

"Atlantic"  .  "Pacific" 

Phonolite,  Phonolite-  Rhyolite 

tephrite,  Basaltic-  Dacite 

tephrite,   Feldspar  Andesite 

basalt,  Nephelene  Basalt 
basalt,  Leucite  basalt, 
Essexite,   Monchiquite 

Just  across  the  arc  of  the  Carpathians  from  the  Bohemian 
Mittelgebirge  is  a  typical  province  of  the  "Pacific"  magma, 
as  was  pointed  out  by  Judd  in  the  studies  which  first  drew 
attention  to  the  fact  that  provinces  of  special  igneous  types 
might  be  distinguished. 

The  choice  of  the  terms  Atlantic  and  Pacific  as  applied 
to  magmas  formed  within  or  about  the  oceans  bearing  these 
names,  has  proved  an  unfortunate  one,  as  has  been  pointed 
out  by  a  number  of  petrologists.  With  the  earth's  surface 
so  divided  into  two  great  provinces,  examples  of  both  Pacific 
and  Atlantic  rocks  are  to  be  found  in  each,  while  there  are 
others  which  seem  to  indicate  gradations  from  one  to  the 
other,  and  these  have  been  termed  "Predazzic"  after  the 
well  known  occurrences  near  Predazzo  in  the  Austrian 
Tyrol. 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY       161 

Based  on  the  contrasted  groups  of  rock  types  represented 
by  the  Bohemian  Mittelgebirge  and  the  Andes,  the  chemical 
characteristics  of  each  group  are  brought  out  in  the  dia- 
grams of  Fig.  78,  where  they  may  also  be  compared  with  the 


AVERAGE  IGNEOUS    ROCK 


PACIFIC  IGNEOUS  ROCK 


PREDAZZIC  IGNEOUS    ROCK 


ATLANTIC  IGNEOUS   ROCK 


FIG.  78. — Comparison  of  the  average  composition  of  Pacific,  Predazzic  and 
Atlantic  rock  types  with  the  average  igneous  rock 

composition  of  the  average  igneous  rock.  It  will  be  noted 
that  the  Pacific  type  is  richer  in  silica,  but  poorer  in  the  al- 
kalies and  in  iron,  lime  and  magnesia  than  is  the  correspond- 
ing Atlantic  type.  The  Pacific  type  is  obviously  of  the  two 
much  nearer  to  the  composition  of  the  average  igneous  rock. 
It  was  early  pointed  out  that  within  many  districts  where 


162      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

there  have  been  successive  eruptions  of  lava,  the  earliest 
possessed  an  intermediate  or  near-average  composition  gen- 
erally corresponding  to  andesite,  that  later  more  siliceous 
lavas  were  erupted,  and,  still  later,  less  siliceous  lavas  which 
were  either  richer  in  soda  or  in  the  constituents  iron,  lime 
and  magnesia. 

We  shall  here  class  together  under  the  name  of  "ande- 
sites"  the  lavas  which  bear  that  name  and  together  with 
them  their  near  relatives,  the  dacites,  trachytes,  and  rhyo- 
lites,  and  designate  them  as  salic  lavas  because  of  their  rich- 


FIG.  79. — Comparative  composition  of  andesitic,  basaltic  and  feldspathoidal 

lavas 

ness  in  silica  and  alumina.  The  later  types  are  by  contrast 
basaltic  (femic)  or,  less  commonly,  feldspathoidal  (alkalic). 
Composites  were  prepared  from  269  analyses  of  "andesites" 
as  above  broadly  defined,  239  basalts  and  149  feldspathoidal 
lavas,  the  last  mentioned  usually  containing  either  leucite 
or  nephelene  among  the  mineral  constituents.  These  lava 
composites  have  supplied  the  comparative  diagrams  of  Fig. 
79.  With  them  is  shown  in  dashed  outlines  for  purposes  of 
comparison  the  composition  of  the  average  igneous  rock. 
From  these  diagrams  it  appears  that  the  earlier  eruptions  in 
the  sequence  so  often  observed  approach  closest  to  the  in- 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY       163 

termediate  or  average  magma,  but  they  are  lower  in  soda 
and  in  the  femic  constituents,  iron,  lime  and  magnesia. 
Had  we  left  out  of  the  composite  the  dacites  and  rhyolites 
which  are  so  generally  of  somewhat  later  eruption  than  the 
andesites,  the  resemblance  of  the  "andesite"  magma  to 
that  of  the  average  igneous  rock  would  have  been  much 
closer. 

It  has  been  noted  also  that  in  a  number  of  districts  the 
low  silica  lavas  of  later  extrusion,  whether  these  be  basalts 
or  feldspathoids,  when  they  have  issued  in  association  with 
mountain  arcs  have  exuded  from  vents  farther  back  toward 


SANTO  AMBRYM 


Andesitic 
Lavas 


Scale       *"""•" 

Section  across  arc  of  New  Hebrides  (Mawson) 
FIG.  80 

the  rear  of  the  arc  than  the  line  of  andesite  extrusions.  This 
relationship  of  position  has  been  well  established,  for  ex- 
ample, for  the  Malayan  arc,  in  which  a  zone  of  volcanoes 
which  have  extruded  leucitic  lavas  lies  to  the  rear  of  the 
great  andesite  zone  of  the  arc.  The  same  has  been  shown 
to  be  true  for  the  island  of  Japan,  as  clearly  brought  out  in 
the  map  by  Iddings  of  the  distribution  of  Japanese  lavas. 
Mawson,  has  added  an  additional  example  from  the  New 
Hebrides  (Fig.  80). 

When  now  we  plot  upon  a  map  of  the  world  the  places  of 
eruption  of  the  andesitic  and  of  the  more  highly  differen- 
tiated and  generally  non-andesitic  lava  types,  most  interest- 
ing geographical  relationships  are  disclosed.  Were  we  to 


164      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

include  with  the  lavas  the  intrusive  igneous  rocks  and  all 
lavas  of  whatever  age,  as  has  generally  been  the  custom, 
our  map  would  be  hopelessly  complex  and  there  would  be 
no  means  of  bringing  the  nature  of  the  volcanic  eruptions 
into  relationship  with  those  movements  of  the  earth's  crust 
which  have  given  it  its  present  facial  expression.  It  is  for 
this  reason,  we  believe,  that  so  much  confusion  has  arisen 
in  the  treatment  of  petrographical  provinces.  In  our  gen- 
eral map  (fig.  81)  we  have  therefore  left  out  of  considera- 
tion the  intrusive  igneous  rocks  and  have  included  only 
those  lavas  which  were  erupted  since  the  beginning  of  the 
elevation  of  the  mountains  in  late  Cretaceous  time  at  or 
near  the  close  of  the  long  Mesozoic  era  of  geological  history. 
The  first  observation  which  is  sure  to  be  made  from  this 
map  is  that  andesites  are  associated  chiefly  with  the  moun- 
tain arcs,  and  that  they  do  not  appear  to  have  been  formed 
to  any  considerable  extent  in  connection  with  the  contrasted 
portions  of  the  earth's  face,  within  which  the  adjustments 
have  been  secured  largely  through  block  faulting  rather  than 
folding.  Eruptions  of  andesitic  lava  appear,  furthermore, 
to  be  most  general  and  constant  for  those  arcs  which  are  to- 
day rising  on  the  borders  of  the  arcuate  mountain  regions, 
and  from  which  in  consequence  the  eruptive  history  may  be 
assumed  to  be  hi  its  beginnings.  This  is  notably  the  case 
for  the  region  of  the  Dutch  East  Indies  and  its  extension  to 
the  eastward  and  southward  into  what  has  well  been  called 
Oceania.  In  the  case  of  those  arcs  which  have  advanced 
farther  in  their  development  we  find  in  addition  to  the 
andesites,  basaltic  and,  rarely,  felspathoidal  lavas  also,  mag- 
ma types  which  have  quite  generally  by  petrologists  been 
ascribed  to  a  later  differentiation  within  an  originally  ande- 
sitic magma. 

Is  there  in  the  process  of  anticline  evolution  with  attend- 
ant fusion  of  shale  beneath  the  generally  competent  lime- 
stone member,  any  apparent  reason  why  the  lavas  should 
appear  in  the  proportions  and  in  the  sequence  which  has 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY      165 


166      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

been  described — should  they  in  the  first  instance  be  largely 
of  intermediate  composition,  and  later  develop  more  sharply 
differentiated  types?  The  upper  section  of  Fig.  82  has  been 


MAGMA  CHAMBER 

I  N 

CORE  Of  FOLD 


MAGMA  CHAMBERS 

AFTER 

RENEWED  FOLDING 


Idealized  Successive  Sections  to  Illustrate  Splitting  of  Original 
Andesjtic  Volcanic  Magmas  into  Basaltic  or  Feldspathoid  Types 

FIG.  82 

based  upon  one  of  the  experiments  by  Willis  where  artificial 
strata  were  so  prepared  as  to  represent  shale  layers  beneath 
a  competent  limestone  formation  and  were  subjected  to 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY       167 

tangential  compression  from  a  direction  at  the  left  in  the 
figure.  To  this  experiment  we  have  added  the  labels  to 
represent  the  relative  positions  of  the  beds  within  the  series 
on  the  assumption  that  the  strata  were  laid  down  during 
a  transgression  of  the  sea,  with  the  superior  limestone 
separated  from  the  average  shale  by  an  intermediate  zone 
of  calcareous  material  having  somewhat  greater  rigidity  as 
well  as  a  higher  fusing  point  than  the  average  shale.  Simi- 
larly the  layers  of  average  shale  grade  downward  into  sand- 
stone through  a  zone  of  siliceous  shale,  likewise  more  diffi- 
cultly fusible  than  the  normal  shale. 

In  this  experiment  a  partial  lifting  of  the  load  by  the  com- 
petent layer  in  the  anticline  is  indicated  by  the  fact  that 
the  shale  beneath  has  suffered  fracture  only  within  a  zone 
near  the  axial  plane  of  the  fold.  Provided  temperature  and 
water  content  are  here  sufficiently  high,  a  magma  chamber 
may  be  formed  within  the  space  where  the  shale  has  been 
fractured,  and  of  this  the  superior  portion  will,  as  appears 
from  the  section,  approximate  the  average  composition  of 
the  shale  formation,  which  is  also  that  of  the  average  ig- 
neous rock.  Somewhat  later  fusion  of  the  fractured  por- 
tions of  the  siliceous  shale  in  an  inferior  position  may  be  ac- 
complished, and  this  magma  being  of  lower  density,  should 
in  time  through  convectional  currents  either  mingle  with 
the  andesitic  magma  above  or  else  rise  through  it  to  the 
upper  portion  of  the  chamber.  In  either  case  the  result  will 
be  to  bring  to  the  portion  of  the  chamber  which  can  be  con- 
nected with  the  earth's  surface,  a  lava  more  siliceous  than 
that  which  first  occupied  this  position.  If  lavas  are  con- 
nected up  to  the  surface  through  a  conduit  rising  from  the 
magma  chamber,  the  order  of  eruption  would  then  be 
andesites  followed  by  the  more  siliceous  dacites  or  rhyolites. 

In  the  process  of  folding  the  initial  anticline  is  first  rela- 
tively symmetrical  and  increases  in  competence  up  to  the 
stage  of  underturning,  where  it  quickly  loses  competence 
but  increases  its  rigidity  in  respect  to  transmitting  the 


168      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

stresses  forward.  The  consequence  is  that  the  deforming 
compressional  stress  when  the  underturning  has  been  ac- 
complished tends  to  be  carried  on  through  the  under- 
turned  arch  and  rears  a  second  but  generally  less  pro- 
nounced anticline  at  the  back  of  the  first,  and  still  later  in 
succession  perhaps  a  third  or  even  a  fourth.  This  order  is 
amply  illustrated  by  practically  all  of  Willis'  experiments, 
which  do  not  however  permit,  owing  to  the  rigidity  of  the 
piston  in  the  apparatus,  that  possible  anticlines  should  de- 
velop at  the  front  as  well  as  the  rear  of  the  original  arch, 
but  with  the  front  anticlines  the  smaller.  There  is  reason 
to  believe  that  in  rock  strata  both  these  types  of  sec- 
ondary anticlines  may  develop  even  though  the  ex- 
periments bring  out  the  mechanical  possibility  only  of 
those  which  form  at  the  rear.  The  effect  of  such  a  second- 
ary anticline  formed  in  the  later  stage  of  anticlinal  develop- 
ment at  the  rear  of  the  original  arch,  would  by  its  lifting  of 
the  load  in  the  new  position  tend  to  extend  the  original  mag- 
ma chamber  upward  at  the  back  into  the  calcareous  shale 
immediately  beneath  the  competent  member.  Such  layers 
upon  fusion  should  develop  lavas  lower  in  silica  than  the 
average  lava  and  richer  in  lime  and  magnesia  and  probably 
in  iron  as  well.  These  lavas  belong  in  the  classes  of  basaltic 
and  feldspathoidal  magmas  and  are  of  notably  greater  den- 
sity. If  gravitational  effects  in  convection  currents  produce 
either  a  mingling  of  these  lavas  with  the  earlier  andesitic  or 
rhyolitic  lavas,  or  settle  down  through  them,  the  effects  will 
be  the  emission  at  the  surface,  first,  of  basaltic  or  felds- 
pathoidal lavas,  or  both,  followed  by  more  acid  andesitic  or 
rhyolitic  extrusions.  If  the  extension  of  the  magma 
chamber  becomes  connected  with  the  earth's  surface  through 
a  newly  opened  conduit,  the  later  extrusions  should  reach 
the  surface  in  a  zone  behind  that  of  the  earlier  extrusions  of 
andesites  and  rhyolites,  as  they  so  often  have  done. 

Of  volcanoes  which  are  directly  associated  with  the  for- 
mation of  arcuate  mountains,  the  order  of  extrusion  of  the 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY       169 

lavas  is  a  matter  of  such  significance  that  it  seems  best  to 
assemble  the  facts  from  many  districts.  This  has  been  done 
in  the  following  table: 

Sierra  Nevadas  oj  California,   Grizzly  Peak. 

1,  andesites,  2,  basalts. 
Cascade  Range,  Northern  Portion. 

1,  rhyolite  and  basalt,  2,  andesite,  3,  rhyolite,  4,  basalt. 
Cascade  Range,  Lassen's  Peak. 

1,  andesite,  2,  quartz  basalt. 
Sierra  Madre,  Mexico. 

1,  andesite,  2,  dacite,  3,  rhyolite,  4,  basalt. 
Isthmus  of  Panama. 

1,  pyroxene  andesites,  2,  rhyolites  and  quartz  latites,  3,  pyroxene 

andesites  and  basalts. 
Rocky  Mountains,  Castle  Mountain  Dist. 

1,  basalt,  2,  rhyolite,  3,  basalt. 
Rocky  Mountains,  Black  Hills. 

1,  andesite  and  rhyolite,  2,  phonolite. 
Rocky  Mountains,  Yellowstone  National  Park. 

1,    dacites    and    andesites,    2,    andesites    and    basalt,    3,    siliceous 

andesite,  4,  basaltic  varieties,  5,  rhyolites  and  basalts. 
Rocky  Mountains,  Silver  City,  Colo. 

1,  andesite,  2,  dacite,  3,  rhyolite,  4,  andesite,  5,  trachyte,  6,  basalt. 
Rocky  Mountains,  West  Elk  Mts. 

1,  andesite,  2,  rhyolite. 
High  Plateaux  of  Utah. 

1,  andesite,  2,  more  siliceous  andesites  alternating  with  pyroxene 
andesites  and  basalts,  3,  rhyolites,  4,  basalts. 
West  Neiv  Mexico  and  Eastern  Arizona. 

1,  rhyolite,  2,  andesite,  3,  basalt,  4,  rhyolite  and  basalt. 
Goldfields,  Nevada. 

1,   andesite,    2,    rhyolite,    3,    andesite,   4,    dacite,   latite    and   rhyolite, 

5,  quartz  basalt,  6,  rhyolite,  7,  olivine  basalt. 
Great  Basin  (part} 

I,  andesite,  2,  trachy-andesite,  3,  andesite,  trachyte,  and  rhyolite, 

4,  rhyolite,  5,  olivine  basalt. 
Central  Cordillera  in  Chile. 

1,  dacite  and  rhyolite,  2,  andesite,  3,  hypersthene  andesite  and 

basalt. 
Graham  Land,  West  Antarctica. 

1,  dacite  and  andesite,  2,  basalt. 
Mitylene,  Asia  Minor. 

1,  rhyolitic  trachyte,  2,  obsidian,  3,  various  andesites  in  succes- 
sion, 4,  basalt. 
Siebenbiirgen,  Hungary. 

1,  andesite,  2,  rhyolite,  3,  basalt. 
Central  Plateau  in  France. 

1,  trachtye  and  rhyolite,  2,  many  varieties  from  trachyte  through 

phonolite  to  basalt,  3,  basalt. 
Japan. 

1,  andesite,  2,  basalt. 

If  now  in  turn  we  take  into  consideration  the  lava  ex- 
trusions which  have  occurred  outside  the  region  of  arcuate 


170      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

mountains,  we  discover  at  once  not  only  that  andesites  are 
relatively  rare  and  that  feldspathoids  are  noticeably  abun- 
dant, but  also  that  the  sequence  of  the  lava  types  is  in  a 
marked  degree  different.  This  will  appear  from  a  few 
examples: 

Northern  British  Isles,  Iceland,  Hebrides,  Faroe  Islands  and 
Portions  of  Greenland. 

1,  dolerites  and  basalts,  2,  andesites  and  dacites. 
Bohemian  Mittelgebirge. 

1,  basalt,    2,    feldspar    basalt,    3,    nephelene    and   leucite    bearing 

lavas,  4,  phonolites. 
Canary  Islands. 

1,  feldspathoids  and  pyroxene  andesites,  2,  basalts. 
Gran  Canaria,  Canary  Islands. 

1,  basalt,  2,  trachyte  and  phonolite,  3,  basalt  and  phonolite. 
Great  Rift  Valley,  Africa. 

1,  phonolite,  2,  basalt  and  phonolite,  3,  feldspathoids  and  basalt. 
Mount  Kenia,  Africa. 

1,  phonolite,  2,  basalt. 
Spitzbergen. 

Feldspathoids  throughout. 
Iceland. 

Very  largely  basalts  but  with  later  soda  rhyolites. 
Azores  and  Madeira. 

Chiefly  basalts  and  feldspathoids. 
Gulf  of  Guinea. 

Chiefly  basalts  with  limburgite  and  andesite. 
Ascension. 

Mostly  feldspathoids. 
St.  Helena. 

Basalt  and  phonolite. 
Tristan  de  Cunha. 

Feldspar  basalt  with  some  hypersthene  andesite. 
San  Francisco  Mt.,  Arizona. 

1,  basalt,  2,  andesite,  3,  basalt. 
South  Victoria  Land,  Antarctica. 

1,  basalt  and  rhyolite,  2,  feldspathoids,  3,  basalt. 

In  order  to  compare  the  Cretaceous  and  Tertiary  vol- 
canic rocks,  which  have  clearly  been  associated  with  arcuate 
mountains,  with  other  lavas  of  the  same  general  period 
which  are  associated  with  major  fault  displacements  and  are 
outside  the  zones  of  arcuate  mountains,  we  have  assembled 
nearly  one  thousand  of  the  superior  analyses  derived  largely 
from  the  tables  of  Washington  and  Iddings,  ranged  them  in 
the  two  groups,  and  from  these  have  in  each  case  prepared 
the  composite  or  average.  In  this  work  the  author  has  been 
assisted  by  Miss  Ellen  Stevenson.  The  results  which  are 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY       171 

set  forth  in  the  following  table  and  in  Fig.  83,  while  they 
show  a  general  resemblance  of  the  two  classes  to  the  so- 
called  Atlantic  and  Pacific  types  of  lava,  yet  reveal  even 
more  clearly  that  the  average  lava  of  the  zones  of  arcuate 
mountains  is  very  closely  in  correspondence  with  the 
average  igneous  rock,  and  that  the  typical  fault-block  lava 
differs  widely  from  this,  chiefly  because  it  is  lower  in  its 


Arcuate  Mtn.(Fold)  Lava 


Average  Lava 


fault  Block.  Lava 


FIG.  83. — Diagrams  to  illustrate  the  average  composition  of  arcuate  moun- 
tain (fold)  lava  and  fault-block  lava  in  comparison  with  the  average 
magma 

content  of  silica  and  notably  higher  in  its  content  in  lime, 
magnesia  and  iron. 

The  lava  analysis  found  to  represent  the  zone  of  arcuate 
mountains  is  a  composite  of  776  separate  analyses  of  rocks 
from  the  Pacific  belt  and  its  extensions  in  the  Malayan 
region.  Europe  has  not  been  included,  because  of  the  vast 
number  of  analyses  which  would  have  to  be  handled.  In 
these  analyses  the  rarer  constituents,  which  are  not  taken 


172      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 


into  account,  are  somewhat  more  abundant  in  the  fault- 
block  type  of  lava. 

Average  Fault  Block 

Magma  Lava 

(Composite  of        (Composite  of 
5602  analyses) 

59.09 
15.35 

6.88 

3.49 
5.08 
3.84 
3.13 


SiOa     , 

( 

Arcuate  Mtn. 
(Fold)  Lava 

Composite  of 
776  analyses) 

5829 

AlaO«     

1697 

Fe20«  

..  1 
....   i 

FeO  

6.94 

MgO    

•  J 

283 

CaO    

606 

Na20 

355 

K20    , 

3.20 

97.84 


96.86 


49.52 
15.89 

11.50 

5.04 
7.27 
4.03 
2.18 

95.43 


If  now  we  endeavor  to  find  in  the  theory  at  which  we  have 
arrived  to  account  for  the  origin  of  the  magmas  associated 
with  block  faulting,  a  reason  for  the  generally  basaltic  or 


DIAGRAM     TO  ILLUSTRATE  A  MANNER   OF  FORMATION 
OF  MAGMA  CMAM6ER5  BY  BLOCK  FAULTING 

FIG.  84 

feldspathoidal  magma  types  characteristic  of  such  regions; 
the  solution  would  appear  to  lie  in  the  special  zones  of  the 
shale  formation  in  which  fracture  occurs  beneath  the 
heavier  formation  when  this  is  thrown  upward  in  the  proc- 


LAVA  COMPOSITION  AND  EARTH  PHYSIOGNOMY       173 

ess  of  block  faulting  (see  ante  Fig.  10  and  Fig.  84).  If  this 
fracturing  occurs  within  the  calcareous  portions  of  the  shale 
formation  in  the  position  closer  to  the  heavier  overlying 
limestone,  the  result  will  be  to  yield  basaltic  or  felds- 
pathoidal  lavas  rather  than  the  intermediate  andesitic  lavas. 
The  field  observations  would  incline  us  to  this  assumption 
if  the  fundamental  premise  is  not  at  fault,  and  if  later  the 
zone  of  fusion  is  extended  downward  toward  the  average 
shale  near  the  middle  of  the  formation,  the  later  extrusions 
would  tend  to  show  more  of  the  andesitic  phases.  The 
order  of  extrusion  would  thus  be  the  reverse  from  that  which 
is  characteristic  of  the  contrasted  type  of  volcanoes  as- 
sociated with  arcuate  mountains. 

LITERATURE 

JOHN   W.  JUDD.    On  the  ancient  volcano  of  the   district   of  Schemnitz, 

Hungary,  Quart.  Jour.  Geol.  Soc.,  1876,  p.  292. 
JOHN  W.  JUDD.     On  the  gabbros,  dolerites,  and  basalts  of  Tertiary  age 

in  Scotland  and  Ireland,  ibid.,  1886. 
F.  BECKER.    Die  Eruptivgebiete  des  bohmischen  Mittelgebirges  und  der 

amerikanischen  Andes,  Min.  Pet.  Mitt.,  vol.  22,  p.  209. 
A.  BARKER.    The  natural  history  of  igneous  rocks,  London,  1909. 
J.  P.  IDDINGS.    Igneous  rocks,  vol.  2,  1913,  part  2,  Occurrence  of  igneous 

rocks,  pp.  343-685. 
H.   S.   WASHINGTON.    Chemical   analyses   of   igneous   rocks,   Prof.   Paper 

99,  U.  S.  Geol.  Surv,  1917,  p.  1201. 
M.  STARK.    Petrographische  Provinzen,  Fortschritte  d.  Min.  Krist.  u.  Pet. 

edited  by  G.  Linck),  vol.  4,  1914,  pp.  251-336  (valuable  for  literature 

also). 

P.  MARSHALL.  Oceania,  Handb.  Reg.  Geol.,  vol.  7,  abt.  2,  1911,  pp.  28-32. 
J.  P.  IDDINGS.  The  problem  of  volcanism,  New  Haven,  1914,  p.  273,  map 

at  end. 


CHAPTER  XIV 

EARTH  THEORIES  IN  RETROSPECT 

ANYONE  who  has  examined  into  the  history  of  the  theories 
of  earth  evolution  must  have  been  astounded  to  observe  the 
manner  in  which  the  unique  and  the  difficultly  explainable 
has  been  made  to  take  the  place  of  the  common  and  the 
natural  in  deriving  the  framework  of  these  theories.  The 
part  of  the  accidental  and  fortuitous  has  been  by  no  means 
a  small  one  in  guiding  the  thoughts  and  the  speculations  of 
those  who  have  dealt  with  the  fundamental  theories  of  the 
universe. 

The  unique  rings  of  Saturn  gave  shape  to  the  nebular 
hypothesis.  The  almost  unique  properties  of  water  near  its 
temperature  of  congelation  was  largely  responsible  for  the 
idea  of  a  molten  earth  core,  long  the  orthodox  doctrine  of 
geological  science.  The  unique  rigid  mass  of  Bohemia  near 
the  arcs  of  Europe  and  that  of  India  near  those  of  Asia, 
largely  determined  the  form  of  the  Suess  conception  of 
arcuate  mountains.  The  centrum  theory  of  earthquakes, 
which  assumed  a  shock  to  go  out  from  a  subterranean 
chamber,  grew  out  of  the  peculiar  experience  of  Robert 
Mallet  in  the  designing  of  heavy  artillery  for  the  Crimean 
War. 

Those  theories  which  have  come  to  receive  general  sup- 
port and  so  have  been  able  to  supplant  earlier  ones,  have 
quite  generally  owed  their  success  to  the  unusual  prestige 
of  their  promulgates  by  reason  of  some  outstanding  piece 
of  investigation,  though  this  may  have  been  in  a  different 
field  from  that  of  the  theory  that  has  been  added  to  the  body 
of  orthodox  doctrine.  This  was  notably  true  of  Laplace  and 

174 


EARTH  THEORIES  IN  RETROSPECT  175 

his  great  work,  the  Mechanique  Celeste,  in  which  the  famous 
nebular  hypothesis  is  contained  as  an  apparently  little  con- 
sidered afterthought  appearing  as  an  appendix.  There 
seems  to  be  a  rather  general  opinion  that  a  large  and  metic- 
ulously precise  piece  of  research  involving  the  assembly  of 
a  great  body  of  data,  necessarily  fits  a  worker  to  reach  cor- 
rect judgments;  though  it  may  well  be  that  his  ability  to 
handle  detail  militates  against  the  forming  of  broad  cor- 
rect generalizations  based  upon  an  ability  to  evaluate  the 
mutual  relationships  of  the  facts  and  principles  involved. 

Once  accepted  by  the  leaders  of  thought,  the  position  of 
a  new  theory  is  one  peculiarly  immune  from  attack.  If  its 
rise  to  prominence  is  in  any  way  sensational,  the  theory  be- 
comes as  it  were,  in  a  measure  canonized  and  clothed  with  a 
quality  of  sanctity.  Attacks  upon  it  are  welcomed  as  little 
by  the  scientific  profession  as  they  are  by  the  promulgate 
of  the  theory.  The  Einstein  theory  of  relativity,  which  is 
just  now  in  the  saddle,  receives  extravagant  praise,  and  the 
voices  of  those  who  point  out  its  fallacies  are  lost  in  the 
thunder  of  applause.  A  great  scientific  weekly  offers  a 
prize  for  the  best  essay  upon  the  theory,  but  with  the  pro- 
viso that  this  essay  be  in  support  of  the  doctrine.  Unfavor- 
able criticism  is  taboo. 

It  is  this  sacrosanct  attribute  of  an  accepted  theory  in 
science  which  alone  can  explain  the  persistence  of  a  false 
doctrine,  even  when  its  errors  can  be  easily  detected  by  the 
application  of  rigid  tests.  How  else  could  the  nebular 
hypothesis  have  endured,  though  so  glaringly  in  error  when 
examined  with  regard  to  the  distribution  of  velocities  of  ro- 
tation, of  total  energy,  and  of  moments  of  momentum? 

No  less  remarkable  is  the  reluctance  after  a  theory  has 
been  proven  to  be  untenable,  to  abandon  the  auxiliary 
theories  which  have  been  founded  upon  the  discredited  one, 
and  which  are  now  found  to  be  out  of  harmony  with  the  cor- 
rected viewpoint.  This  has  been  especially  true  of  the 
theories  which  have  been  based  upon  the  nebular  hy- 


176      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

pothesis.  Though  the  conception  of  a  molten  earth  in- 
terior has  perforce  been  abandoned,  there  has  been  a  note- 
worthy unwillingness  to  accept  the  consequences  and  to  take 
up  de  novo  the  related  and  dependent  problems.  Thus  we 
find  an  impelling  urge  to  assume  in  the  place  of  a  liquid 
earth  interior,  at  least  a  continuous  substratum  of  lava  be- 
tween a  rigid  core  below  and  a  solid  crust  above.  It  is  clear 
that  if  such  a  stratum  is  assumed  those  theories  which 
have  grown  out  of  the  nebular  hypothesis  can  survive  un- 
changed. 

The  theory  of  the  permanence  of  the  ocean  basins  when 
found  to  be  hopelessly  in  opposition  to  the  later  discoveries 
in  fauna!  geography,  instead  of  being  abandoned  by  those 
who  had  supported  it,  was  adjusted  to  the  later  knowledge 
by  throwing  narrow  "bridges"  across  the  oceans,  bridges 
sufficiently  wide  at  least  for  the  animals  to  advance  over 
them  two  by  two  in  the  decorous  fashion  in  which  they  are 
supposed  to  have  entered  the  Noachian  ark.  The  abandon- 
ment of  the  centrum  theory  of  earthquakes  caused  but  little 
change  in  the  then  orthodox  method  of  preparing  seismic 
maps,  which  still  continued  to  be  brought  out  with  their 
"epicenters,"  "isoseismals,"  and  "coseismals,"  though  these 
could  have  no  existence  unless  there  be  a  unique  focus  from 
which  the  shocks  have  emanated. 

It  is  this  quite  natural  reluctance  to  repudiate  long  cher- 
ished doctrines  which  has  made  the  real  transition  from  one 
fundamental  viewpoint  to  another  that  is  to  supplant  it, 
cover  periods  measured  in  decades  rather  than  in  years. 

In  the  foregoing  pages  we  have  gone  out  from  the  as- 
sumption that  several  theories  occupying  a  fundamental 
position  in  the  science  of  geology  must  now  be  abandoned 
as  untenable.  The  nebular  hypothesis  with  its  natural  off- 
spring, the  doctrine  of  a  molten  earth  interior,  we  believe 
the  consensus  of  scientific  opinion  now  places  in  the  cate- 
gory of  theories  which  under  the  application  of  rigid  tests 
have  been  found  wanting.  Upon  the  other  hand,  the 


EARTH  THEORIES  IN  RETROSPECT  177 

dactrine  of  the  subterranean  zone  of  flow  and  the  view  of 
Suess  concerning  the  direction  of  thrust  in  the  evolution  of 
mountain  arcs,  are  theories  rather  generally  accepted  by  the 
geologists  who  have  studied  the  questions.  None  the  less 
we  believe  that  neither  of  these  theories  is  tenable,  and  at 
some  length  we  have  set  forth  the  basis  for  our  belief. 

The  arcuate  island  groups  in  the  Pacific  with  their  at- 
tendant ocean  deeps  are  believed  to  outline  the  positions  of 
developing  folds  within  the  near-surface  strata.  A  uni- 
versal subjacent  reservoir  of  magma  beneath  the  earth's 
outer  shell  being,  as  we  believe,  no  longer  available  to  ac- 
count for  the  source  of  lava  and  its  attendant  gases  and 
vapors;  we  have  sought  for  an  available  source  and  believe 
that  one  is  found  in  the  fusion  of  shaly  sediments  beneath 
rising  anticlines  and  beneath  upthrown  blocks  of  the  fault 
mosaic.  Both  the  lavas  themselves  and  their  attendant 
emanations  find  an  explanation  in  this  theory  of  origin. 
For  the  first  time  an  explanation  is  found  for  the  strictly 
limited  range  in  chemical  composition  of  volcanic  lavas. 

Certain  facts  which  have  hitherto  appeared  as  paradox- 
ical because  their  causes  were  obscure,  have  been  given  a 
relatively  simple  explanation.  The  long  established  fact  of 
the  occurrence  of  block  mountains  within  growing  mountain 
belts  has  required  that  these  belts  should  be  characterized 
by  tensional  stress  conditions,  although  the  evidence  for 
such  a  system  was  notably  wanting.  It  has  here  been 
shown  that  the  conditions  are  in  fact  the  reverse  of  those 
that  had  been  supposed,  and  that  the  system  of  stresses 
within  the  zone  of  rising  mountains  is  not  tensional  but 
compressional,  and  that  the  sea-floor  is  pushed  up  onto  the 
seaward  slopes  of  the  rising  mountains  by  an  amount  more 
than  sufficient  to  compensate  for  the  expansion  of  the  sur- 
face. Thus,  moreover,  is  found  a  cause  for  the  erection  of 
mountains  upon  the  borders  of  the  sinking  ocean  basins. 

Again,  the  more  recent  developments  of  seismology  have 
been  drawn  upon  to  account  for  the  great  outflows  of  lava 


178      EARTH  EVOLUTION  AND  ITS  FACIAL  EXPRESSION 

within  block-faulted  regions.  Lastly,  a  comparison  of  the 
lavas  which  have  been  forced  up  to  the  surface  in  connec- 
tion with  the  growth  of  arcuate  mountains,  with  those 
erupted  in  connection  with  major  fault-block  adjustments, 
has  brought  out  the  fact  that  these  lavas  are  essentially  in 
contrast;  and  that  the  former  approach  in  composition  to 
the  average  igneous  rock,  whereas  the  latter  type  is  of  dif- 
ferent composition  and  to  a  notable  degree  more  differen- 
tiated and  variable. 

A  general  conclusion  which  has  been  drawn  from  these 
studies  is  that  it  has  been  customary  greatly  to  overestimate 
the  span  of  geological  time,  and  this  largely  because  of  the 
accident  of  location  of  those  who  have  studied  geological 
processes.  Had  the  early  universities  been  located  within 
the  Pacific  area,  rather  than  about  the  Atlantic;  had  geol- 
ogists made  their  investigations  within  those  belts  of  the 
earth  which  are  undergoing  rapid  change;  the  orthodox  view 
concerning  the  time  which  has  been  necessary  for  the  ac- 
complishment of  the  past  geological  changes  would  have 
been  found  to  be  a  fraction  only  of  that  which  it  is  now 
supposed  to  be. 


4021 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


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